If I suspected collusion or soft play, particularly based on player history, then I would penalize. But I don't feel I'd be in the right to force the player just because he has a lot of chips. Perhaps he should call based on the odds, but that doesn't mean that he must call.
I personally wouldn't check his cards (without prior suspicion of collusion), but I'd have no hesitation suggesting that his play is a bit unorthodox and probe for an explanation. If he answers "he's my friend and I didn't want to see him bust", he'll be receiving at least a warning if not a penalty right there. On the other hand, if he answers "I had a crap hand and didn't want to double him up", I don't see what would have given us the right to play his hand for him and force a call.
The more interesting situation that I've heard about is along the lines of: "I am chip leader and by keeping this small stack in, I can continue to put pressure on all the remaining stacks at the table, who are all also short and playing scared, hoping that this 9th player will bust. They can always try to take the short stack out themselves if they want but I am not going to waste my chips." There was an interesting debate on this situation, and I'm not sure if a consensus was ever reached. Personally, I don't mind that strategy that he claims to be using -- as long as he's taking advantage of the stack situation and not favouring that individual in particular, he's earned those chips so who am I to tell him when to call or fold.