One evening of use will not have any detrimental effects on your trailer with no more seating capacity that it has. Personally, my only response would be "I'll help you hook it up". I guess with my mindset, it would have never entered my mind to have the neighbor sign anything. I have read about the sue happy people but don't think I have ever met one of them. Tell them congratulations, and invite them over for your next cookout. If someone doesn't make an effort, nobody would have good neighbors.