I'd never met him previously but am a generous guy and thought I would try help out a fellow poker professional.
If the backer thinks staking a scammer/compulsive problem gambler is profitable enough to risk his reputation and the profits off Marc's stake, that's his perogative and you should call him out on it as implied above. If he's really a decent guy he'd know its now on him to deal with his horse's degeneracy when the horse starts making deals on his behalf without consulting him (IE knowing that this is a prominent risk once agreeing to back him for EPTs and more likely than not to have happen).
As you did the same thing (knowingly and unnecessarily engaged in business deal with known scammer/degen), it's on you as well.
It's Marc's responsibility to pay in full. If he doesn't, it's on you and the backer to come to some sort of agreement and put it to rest. Everyone who knowingly takes on a lot of risk needs to be prepared to deal with the fallout when the inevitable comes to fruition.
EDIT - To be clear, I don't think the backer is always on the hook in every situation...but it's pretty clear that he'd be similarly as big an a-hole with similarly bad business practices if he didn't try to help make things right in this type of spot.