Is this just standard lease boilerplate that my friend shouldn't be too worried about?
One should never disregard a clause under the belief that it is "standard boilerplate".
The purpose of written contracts, such as a lease, is to supersede --or at least formalize-- ordinary or standard relations. Accordingly, parties to a contract should always expect for contractual entitlements to be exercised by a counterparty, since that is precisely what the agreement allows (except for clauses which contravene legislation).
How much should my friend be concerned about this clause?
Your friend should decline a contract which contains clauses he finds unacceptable.
Although clauses might be stricken in court as unconscionable, the clause at issue would reduce the grounds on which your friend may allege unconscionability and object to the landlord's arbitrariness. For instance, section 1932, paragraph 2, of the California Civil Code provides that the hirer (here a tenant) may prematurely terminate the lease:
[w]hen the greater part of the thing hired, or that part which was and which the letter had at the time of the hiring reason to believe was the material inducement to the hirer to enter into the contract, perishes from any other cause than the want of ordinary care of the hirer.
This provision is more related to a tenant's claim of constructive breach of contract, but it also applies to your friend's situation: Agreeing to landlord's full discretion implies that most likely this landlord's occurrences did not affect, or would not have affected, the material inducement(s) to the tenant. The rationale is that otherwise the tenant would not have knowingly and willfully waived the protections that preserve such inducement(s).
Consequently, your friend would have a harder time alleging future changes are unreasonable than if he rejected, or at least constrained the scope of, the clause at issue.