Well if you aren't in a committed relationship with anyone, then why not? That guy that you're somewhat seeing doesn't really have a right to stop you if he says that what you two have isn't committed or official or whatever.
But just to warn you: If the guy that you're seeing gets jealous, then it's probably best that you stop doing stuff with his best friend or talk to him. Remind him that you guys are not committed.
Things like this can always get a tricky and really depends on the situation, who's involved, and what everybody's emotions are. I can never tell you what to do because I don't have a feel for what's going on. My best advice would be to do what you want to do and see how it goes. Playing it by ear can be "dangerous" but your intuition knows more than you think.
