Hmm... it’s hard to say what would be the best plan of action to take. You’ve been told the one is cheating on the other, you don’t know either that well, but you want something to be done.
One thing I think you should remember is that you were told by the best friend of the cheater. You’ve not seen her cheating on her boyfriend, and don’t really have any evidence. When it comes down to it, it’s the rumours that she’s not a saint, and your word against hers.
You don’t know the person being cheated on that well, but would you like to become friends with him? By telling him, it could make him trust you more and want to become your friend.
Also, while you don’t know them too well so shouldn’t worry about what they think, would your own friends be against the idea of telling the boyfriend that he is being cheated on? If you involve yourself, it may have repercussions on you which you should be aware of.