Look, you're willing to help these folks out, and you're not asking them to pay excessive money for it. All you're asking for is a simple document that would protect you from liability in case of questions in the future. Not only is that reasonable, it would be risky to do the work without it. Imagine how it could look - former employee, possibly a disgruntled employee, trying to break into the system.

What you're asking for is a small matter, one that would protect you while not costing the company anything more than a few moments of typing. If they're not willing to be bothered with it, then perhaps they don't really need your services so desperately.