Let me get this straight, because I'm getting confused here, there is '0' proof for or against him being on the hook for his credit, but just for the sake of the argument let's assume that it is his personal credit that is linked to the merchant account (that would be spectacularly stupid, but ok).
You are suggesting that he take out a private loan even though he's already committed his savings to this venture to the tune of $17K in order to protect his credit rating?
Worst case he won't be able to borrow money for a while, and he might not be able to get a mortgage or other financing.
Would you borrow money in order to be able to borrow money later? It makes very little sense to me, to pay back $17K privately (so that's after taxes) means that you'd have to be earning roughly double that just to get rid of this debt, at a regular salary and expenses that comes down to at least two years before you can get rid of that debt, not counting the interest.
How does that make any sense at all, is your credit rating worth 20K and two years of your life to you?