This always seems to be the answer, but...
Talk to the player first.
I'm assuming you have some means of contacting your players outside your normal game time, if only to set up game or let each other know of cancellations or emergencies. Send your player a message, something along the lines of "Hey, I've noticed that you seem dissatisfied at game lately. Is there something going on you're not happy with? I want to make sure our game is fun for all of us, so please let me know if you have any concerns."
See how he responds to that. If he responds with a list of concerns, use those as a starting point. If they're things you can address, try to do so. If you think his problems are irreconcilable with your game and/or the other players, then say that, politely, and suggest that he might want to look for a different game to join.
If his response is neutral or brushes you off (such as "nothing's wrong, see you next game"), then you can let it go for a session or two. See how he behaves, and whether he raises the issue himself.
Ask another player to initiate communication
If you're not comfortable approaching the problem player directly (or don't have the means to do so), you can speak again with the person who told you this player wants to leave. It sounds like the go-between is friendly with him, so you can ask the go-between to, next time the player brings up the issue of wanting to leave the campaign, tell him to talk to you about it.
Don't use the go-between to actually convey messages like "I heard you want to leave, what did I do wrong?". That kind of thing usually gets lost in communication, and it puts the second player in an awkward spot. Just ask him to pass on that you're open to hearing your players' concerns and that the player is welcome to talk to you if he has a problem.
Don't jump the gun
Either way, don't rely on hearsay to drop a character and his player from the party. Don't allow the other players to sacrifice this guy's character unless you have his explicit buy-in (or if it comes up naturally in-game and the player himself is all for it). Don't assume that he meant what he said about wanting to drop, either - I've had players complain about my game in moments of frustration, but when I ask them if there's something I can do better, they reassure me they're having fun and were just briefly frustrated.
TL;DR: Communication is your friend!
Talk to the player. Follow his lead, and don't make assumptions based on what other people are telling you. If you handle this issue with grace and good will, then it's highly unlikely the other players will see a reason to stop playing a game they enjoy just because their friend did.
Why not introduce some major incident that lets your PCs go unwatched, thus with the ability to free themselves and then help fight the incident?
Considering the fact that this is an historical campaign, you could start something big that didn't make it to the history books, and it could be thanks to your PC.
This way, your PC redeem themselves, AND write history!
The incident can even be Bob's character's doing, this way your PC:
- mess with history,
- fix it,
- stop Bob's character and may even kill him, to their great satisfaction,
- don't have the feeling that you saved them, in fact if you play this well, your only part will be to give them a chance to escape.
If you are concerned about how Bob will react, consider that you had a long talk with him, and he had several chances to correct his behavior. Clearly he had no respect for others' fun, so why bother?
Best Answer
To me the problem seems to be that the other players think that they gained treasure as a team and divided it up according to some measure of 'fairness' and 'need.' So when one player leaves, they think that their joint loot is up for redistribution. That's a very pragmatic approach, and perhaps you should discourage it to encourage in-character gameplay (that's assuming you want such a thing).
Assume that the character of the departing player becomes a non-player character. Especially if the split was on good terms, the player might even come back for a couple of sessions, down the line.
Try to explain this 'deal' to your players. Try and make them understand that looting the corpse while it is still warm is not the best choice on the long run.