Inherently, no, there's nothing wrong with dating a partner's friend. About the only general rule in polyamory is that dishonesty is bad. Everything else is up to the individuals involved to determine for themselves.
Considering you started this thread, I take it you're concerned your partner might not like it. There's really only one way to be sure: talk to your partner about it. Listen to any concerns they have, and go from there. You know, that whole "communication" thing everyone's always talking about that no one seems to know how to do.