some friends of mine rented a minivan for about 2 weeks from Alamo out west, and after returning it with no comment from the check-in people 2 weeks ago, now they've received a bill in the mail for some $800 worth of damage to the van. They say that absolutely no damage to the car was done, and the items shown are bogus.

Do any of you have some advice for how to dispute this/make this headache go away/steps to be sure to take for resolving it?

Thanks a lot for your advice!