Presumably, the condominium association covenants prohibit three unrelated people from living in a unit. At face value, your proposed rental violates those standards.
Maybe Fair Housing laws say that condominium associations are allowed to do that, maybe Fair Housing laws don't prohibit such limitations. The main federal Fair Housing law protection that might apply is as follows (there also might be Boston or Massachusetts specific laws on point):
Discrimination in Housing Based Upon Familial Status
The Fair Housing Act, with some exceptions, prohibits discrimination
in housing against families with children under 18. In addition to
prohibiting an outright denial of housing to families with children,
the Act also prevents housing providers from imposing any special
requirements or conditions on tenants with custody of children.
For example, landlords may not locate families with children in
any single portion of a complex, place an unreasonable restriction
on the total number of persons who may reside in a dwelling, or
limit their access to recreational services provided to other tenants.
In most instances, the amended Fair Housing Act prohibits a housing
provider from refusing to rent or sell to families with children.
However, some facilities may be designated as Housing for Older
Persons (55 years of age). This type of housing, which meets the
standards set forth in the Housing for Older Persons Act of 1995, may
operate as "senior" housing. The Department of Housing and Urban
Development (HUD) has published regulations and additional guidance
detailing these statutory requirements.
The occupancy limitations are not clear cut under federal law, however, and arguably apply only in the case of families with push up against occupancy limits because they have many children:
What is considered a reasonable occupancy standard? The Fair Housing
Act specifically allows housing providers to adhere to reasonable
local, State, or Federal restrictions regarding the maximum number of
occupants permitted to occupy a dwelling (42 U.S.C. 3607(b)(1)). Such
restrictions may include property maintenance codes, zoning codes,
minimum floor area requirements, or other similar provisions. These
occupancy restrictions often take into account factors such as the
number and size of sleeping areas or bedrooms, the overall size and/or
configuration of the unit, and/or other physical limitations of the
housing, such as sewer or septic capacity. If a housing provider
allows fewer occupants than would be allowed under the applicable
code, then it could be challenged as discrimination against families
with children.
Protection of the right of unrelated people to live together doesn't really protect "familial status" in the sense of the Fair Housing Act.
But, the landlord, naturally, would prefer not to expend his limited resources in an uncertain test case suing his condo association to determine that its covenants are void because they violate Fair Housing laws, while running up fines and liens on his unit in the meantime, and facing an attorney fee award against him from the condo association if he loses. He would probably not be deemed to violate fair housing laws for doing so.
As a practical matter, you would be better served by looking elsewhere or dropping your proposed roommate.
Flaglink below your question.