It's difficult to summarize the issue in the first letter to the Ethicist this week without losing relevant detail, so I'm simply going to quote it:
I use my furnished condo as a summer home and rent it out the rest of the year. A potential tenant, a lady from Pakistan, seemed ideal except for one thing: the condo's ventilation is not very good, and daily cooking with heavy spices would make the curry smell impossible to remove. I felt guilty rejecting her because of her cooking...
Cohen's answer addresses both moral and legal issues, so this one will as well. The moral problem, which Cohen seems to be implying by example, is one of implicit bias. That is, if the renter had an established policy (even a policy that only the renter was aware of) of never renting to anyone who was likely to leave the aparment with a non-temporary displeasing odor, and did his/her1 best to check whether or not each potential resident was going to cause a conflict with that policy, there would be no problem. Further, this wouldn't be a problem (ethically) even if, in enforcing the policy, it turned out that cases where renting to someone would lead to a violation of it, that someone is a member of an ethnic group other than that of the renter, as long as it truly was the potential odor which motivated the lack of an offer to rent. But going from the data point of "potential tenant is is member of ethnic group x" directly to the conclusion that an odor problem will exist is just begging to give biases an unreasonable sway in decisionmaking.

Legally, whether or not a potential claim is being created (assuming that this is the only home which the renter rents out, as the letter seems to imply) depends entirely (at least for Fair Housing Act purposes, I'm not considering any other Federal law or potential state laws) on whether or not the renter a) made use of a real estate broker or similar service, or b) advertised the rental in a way that indicated an intent to base the choice on an impermissible characteristic (FHA ยง803(b)(1)). The latter seems unlikely from what's indicated in the letter, and I have no basis to opine on the former.