If you don't feel able to talk to her about it (and of course, there's no reason why you should) then yes - it's a HR issue.
They should deal with it in confidence, so the woman doesn't have to know who has made the complaint - they don't even have to tell her that someone has made a complaint, it could simply be treated as something they have concerns about with regard to the cancer treatment your boss's wife is having.
My boss used to have a terrible BO problem, and it wasn't pleasant working in the same office. I spoke to HR about it, they had a word, and lo and behold a clean shirt every day and a bottle of deodorant in the office worked wonders!