Ah, this is different then.
All I can really say is that Bob will have to move on in the end. It is all he can do. Whether or not keeping in contact with the friend is part of the moving on.
It's hard for us to tell what's really going on. The girl could be rebounding. Maybe she had an eye out for the new guy and decided she wasn't satisfied with the relationship, so she ended it.
Confronting them will do nothing, except drive Liz more towards the new guy. The best thing to do is to stay away from both people if at all possible. As cheesy as this saying goes, "Time heals all wounds."
As for you, be there for the poor guy. Break ups suck, especially for the person who still had more emotional attachment in the end. Even moreso when one person has all ready moved on.