I have to argue that maybe he kept it from you because "Bob" respects how you view him. Maybe he's just afraid to let you down. Obviously I don't know the specific dynamics of your friendship, but I could see this being the case. Sometimes it's harder to admit to a friend that you f*cked up.
My advice, I'd ask him about it (in the right setting/moment). Bring it up casually and see how he takes it. If he asks how you found out, just say you had your suspicions and he confirmed. Don't start pointing fingers or anything, but let him know you're there for support (assuming you want to be). If you just ignore it, it will just progressively get worse until you have another falling out. Like @BropolloCreed79 and @TheHansonGoons said, it has to be HIS decision, but that decision is easier to make with the help of those around you. In my opinion, this is something you might want to get in front of.
Really, this depends on if YOU think the friendship is worth it.