If the car was insured it would have been totalled. The car is only worth $3500 retail according to KBB. Are you sure it's worth fixing?

That aside, a deer isn't heavy enough to do significant damage to A pillars. They will probably need some minor realignment, but otherwise, the structural integrity should be comparable after the repair if you're a descent welder.

This is a 15 year old car, the repair is being done under good faith by a person experienced in this type of repair. The waiver isn't a bad idea, but as long are you aren't giving the impression of being a professional mechanic, the owner should not have reasonable expectation of the work being done properly. You can protect yourself further by not charging for the work. If you are compensated for the work, you open yourself up for litigation.

That's my $0.10