Blue Ghost,
I've not been in your situation exactly, but I can empathize with you a wee little bit
What follows is a precise of my own gaming experiences - which I will add, may or may not have anything to do with your situation. Since I'm not fully briefed on your situation, and since sometimes people leave out important little nuggets without meaning too (ie, unaware of the value of said information or they don't realize that it is NOT self-evident...)
1) as a GM, I have come to rely upon ONE major player in my group precisely because he is the "leader" type and is also my best friend. Were he to leave my gaming group for any reason, the gaming sessions would lose a BIG part of its flavor. I don't know if you occupy that role in your friend's life or not - but you may want to think about it further. Do you occupy a specific niche in his campaign *STYLE* that without you, might make the game less enjoyable for him?
2) Social dynamics being what it is - is there perhaps others in your gaming group who have come to appreciate your presense within the group? Oddly enough, if you have G,A,B,C,D & E playing together, and G is happy that you, "A", are playing - the dynamic would be simple enough that without you, the gaming experience is less than it could be. However, lets presume that the G-A relationship is not enough to keep you playing. Suppose B&C are fed up with D's behavior, and are sticking around because they like G and A - and are neutral for E. Pull "A" out of the picture, and suddenly, the entire group becomes destined to fold as others find outside commitments to attend to. Now, look at it from a really convoluted sense <g>...
G is happy with everyone.
A is unhappy with D and wants to leave
B is happy with G & A, but is neutral towards everyone else
C is there because of B
D is there because, well, because <g>
E is there because of B & C.
A leaves. G is unhappy sorta, but the show must go on. B leaves because, without A, it gets WORSE instead of better in his opinion. Besides, without A being around, D gets WORSE. So now A & B are gone. E decides that with B's leaving - he's got no real reason to stick around.
What I'm saying is that the law of unintended consequences result in the destruction of a viable gaming group. One of my rules as a GM and as a host of gaming at my house - all newbies must be vetted (approved of) by the long standing members. If for any reason, he/she gets blacklisted, they are told "sorry, but that's how it goes". Stats on my gaming group? One has been with me since 1980. Another since 1983. A third, since 1986 (and I married HER!). One player until he died of a sudden heart attack, had been with us since 1990. We've had some come and some go - but in general, we've survived stupid stuff <g>.
My suggestion? Talk to your GM buddy and let him know "Hey, I really LIKE being around you and were it not for you, I'd probably not be here. But that dunce of a player without social graces is ruining my enjoyment to the extent that I really can't stick around. I'm letting you know this because if I'm ready to leave, who else might be? All things considered, I've agonized over losing your friendship, and I now realize that regardless of whether I stay or not, you and I will retain our friendship - it is too valuable to not keep it! On the other hand, rather than stick you with an ultimatum to the effect either he goes or I go, I'm going to bow out for a short time. If or when the other player gets his act together, or he's no longer part of the crew, let me know. Also, if I find a game I'm enjoying, I will be sure to ask if you can join too"
Emphasize why you're going, emphasize the value of your friendship, and then emphasize the fact that you're not trying to FORCE him into choosing between you or the other friend of his. MORE importantly, don't let his seeming reluctance to kick the other guy out reflect badly against his value of friendship to you. I've been in the middle of such "politics" myself (I've GM'd since 1980) and it is no fun. Once, I told an entire gaming crew of roughly 9 people "Ok, I've had it, get the HELL out of my house. You all know my views on hospitality and my wife and I have had words over violations of common courtesy - but this takes the cake. There will be NO gaming and unless you are personally invited back, you are no longer welcome within the crew"
The cause of the blow up? First - it should be noted that we EAT dinner together and socialize in addition to gaming. Secondly, players were taking things personally that they had no business taking personally. Thirdly - there was so much unpleasantness on a personal level that it got badly out of hand and one day, I just snapped thinking "I would rather read WAR AND PEACE than GM another game for this crew!". Roughly half of the game crew got weeded out after that fiasco.
Maybe my story isn't all that common, or maybe it IS common. Gaming buddies from college days are still gaming buddies today. My wife got involved in both fencing and gaming to spend time with me - and is now one of the better roleplayers I know. She has NEVER (well, until GURPS MAGIC) read a single game book. She is a ROLEPLAYER instead of roll player. Barracks rules lawyer is as foreign to her as breathing vacuum is to most carbon based lifeforms
So, good luck with your situation, and as ever - my email is always available for private comments if needed.
Hal