Yeah, drunks always say, "C'mon, I won't hurt it... and if I do, I'll pay for it." The problem, of course, is that they are drunk. Legally, they are not competent to enter into any contract, verbal or otherwise. They are drunk. Intellectually, they can't comprehend that the replace cost of your guitar is in the thousands of dollars. They are drunk. And like Pierre said, "You should not have let them borrow it in the first place" because the onus is on you to make a smart decision about your instrument. They are too drunk to be smart.

"No, no, no, Hell No!"

:)