As long as you're clear as to what role you played in the project, it's fine to put anything you worked on in your portfolio.
If you're a "production person" or "front end developer" it's not an issue, however if on the other hand you're selling yourself as a "creative professional" it's a bad idea. The only exception might be if you used a super minimal theme and re-designed it from the ground up so it doesn't look anything like the original.
I have listed a site which uses a purchased/premium template before. However, I made it quite clear that I did not do the design work that I simply did the front end/backend coding and that someone else did the design work.
I wasn't trying to sell myself as a designer, though...
As long as you/he states the kind of work done (slicing, theming, adding plugins etc) I think it's pretty fair.