Number one, your friends need to speed up. Snake's attack are fairly laggy on shield. The frame advantage is there for people to grab snake before Ftilt 2 comes out. The after lag of Ftlit 2 on shield is certainly punishable. Utilt is laggy enough that you can grab it, but you have to be wary of the shield push. In that case, dropping shield into a dash attack works for virtually every character.
Your friends probably need to wisen up about how they approach snake in the air. He is at is weakest at that spot. He has so few options to landing. Airdodge, grenades, 2nd jump, or aerial. They should figure out that Snake wants to get back to the ground, but by using only those methods. From there, you need to make choices that will cover a few of those options.
http://www.youtube.com/watch?v=7q2DdBNajvo&feature=related Numerous times in this video, M2K spaces a jump -> Uair so that had ally not airdodged, he would have gotten hit by Uair. However, M2K was in the right postion after his attack that he could punish an airdodge with Nair. The same has to be done for other options snake has in the air. The idea of attacking to provoke a reaction from snake, but should the attack fail, you launched your attack in a way that you do not get punished for it, and you can punish Snake for doing something.
Even if I am not MK, but I am playing Falcon, I can try to provoke a reaction from Snake by jumping near him, but out of the attack of retaliation from Snake. This way, if he does something, I can follow-up on it.
Nearly every character has some way to punish Snake's tilts. So long as you are not hit, you can punish snake for it. The biggest way is like I said by trying to use a dash attack. Most dash attacks cover a large area in a sort amount of start-up lag.