The Ontario Residential Tenancies Act says that:
64 (1) A landlord may give a tenant notice of termination of the
tenancy if the conduct of the tenant, another occupant of the rental
unit or a person permitted in the residential complex by the tenant is
such that it substantially interferes with the reasonable enjoyment of
the residential complex for all usual purposes by the landlord or
another tenant or substantially interferes with another lawful right,
privilege or interest of the landlord or another tenant.
There are statutory provisions whereby a landlord is prohibited from himself interfering with a tenant's quiet enjoyment of the premises, which would not be applicable. Given just this, you may, but are not required to.
The Conveyancing and Law of Property Act (23)(1) says:
In a conveyance made on or after the 1st day of July, 1886, there
shall, in the cases in this section mentioned, be deemed to be
included, and there shall in those cases be implied, covenants to the
effect in this section stated, by the person or by each person who
conveys, as far as regards the subject-matter or share thereof
expressed to be conveyed by him, with the person, if one, to whom the
conveyance is made, or with the persons jointly, if more than one, to
whom the conveyance is made as joint tenants, or with each of the
persons, if more than one, to whom the conveyance is made as tenants
In a conveyance for valuable consideration, other than a mortgage, the
following covenants by the person who conveys, and is expressed to
convey as beneficial owner, namely, covenants for...ii. quiet
enjoyment...according to the forms of covenants for such purposes set
forth in Schedule B to the Short Forms of Conveyances Act, being
chapter 472 of the Revised Statutes of Ontario, 1980, and therein
numbered 2, 3, 4 and 5, subject to that Act.
In other words, if the lease doesn't say anything about quiet enjoyment, there is an implied covenant whereby "the landlord, by letting the premises confers on the tenant the right of possession during the term and impliedly promises not to interfere with the tenant’s exercise and use of that right to possession during the term". The implied covenant pertains to interference by the landlord, not to interference by other tenants (see this analysis). Again, you would have a right to evict a noisy tenant, but not an obligation.
However, the Canadian Human Rights Act(6) states that:
It is a discriminatory practice in the provision of commercial
premises or residential accommodation (a) to deny occupancy of such
premises or accommodation to any individual, or (b) to differentiate
adversely in relation to any individual, on a prohibited ground of
where 3(1) states:
For all purposes of this Act, the prohibited grounds of discrimination
are race, national or ethnic origin, colour, religion, age, sex,
sexual orientation, gender identity or expression, marital status,
family status, genetic characteristics, disability and conviction for
an offence for which a pardon has been granted or in respect of which
a record suspension has been ordered.
The Canadian Supremacy Clause (52(1) of the Constitution Act, 1982) says that:
The Constitution of Canada is the supreme law of Canada, and any law
that is inconsistent with the provisions of the Constitution is, to
the extent of the inconsistency, of no force or effect.
The Constitution of Canada includes (a) the Canada Act 1982, including
The Constitution act (15)(1) also states:
Every individual is equal before and under the law and has the right
to the equal protection and equal benefit of the law without
discrimination and, in particular, without discrimination based on
race, national or ethnic origin, colour, religion, sex, age or mental
or physical disability.
So under the Canadian Constitution, the national anti-discrimination requirement is superior to the provincial RTA. Since no provincial law actually requires eviction of a tenant with a noisy guest, there is no conflict.
Ontario also has an anti-discrimination law (The Human Rights Code):
Every person has a right to equal treatment with respect to services, goods and facilities, without discrimination because of
race, ancestry, place of origin, colour, ethnic origin, citizenship,
creed, sex, sexual orientation, gender identity, gender expression,
age, marital status, family status or disability.
(1) Every person has a right to equal treatment with respect to the occupancy of accommodation, without discrimination because of race,
ancestry, place of origin, colour, ethnic origin, citizenship, creed,
sex, sexual orientation, gender identity, gender expression, age,
marital status, family status, disability or the receipt of public
(2) Every person who occupies accommodation has a right to freedom
from harassment by the landlord or agent of the landlord or by an
occupant of the same building because of race, ancestry, place of
origin, colour, ethnic origin, citizenship, creed, sexual orientation,
gender identity, gender expression, age, marital status, family
status, disability or the receipt of public assistance.
Right must therefore refrain from hassling Left.
A problem could arise if there is a lease clause whereby you guarantee tenants a noise-free environment and promise to remove noisy neighbors. In that case, Right could sue you, and you would have to defend yourself by reference to the fact that Canadian and provincial law prohibits you from pursuing eviction in this case.
An alternative would be to ask Left to not have the noisy individual over. But that would not be a wise choice. The anti-discrimination laws are not stated as protections against tenants who have the protected characteristics, indeed all tenants have some sex, religion, race etc. You can't start eviction proceedings because the tenant is white, black, Catholic, or atheist, and you likewise can't evict a person for having a guest who is white, black, Catholic, or atheist. You also can't "have a talk" with a tenant – creating a "poisoned environment" – because they or their guest are white, black, Catholic, or atheist. Having a mental disability is included in this set.
All that said, this is clearly a case that needs to be looked at by an Ontario landlord lawyer.