I can see both sides of the coin but lets face it

1 your job is not to risk a "potential" dangerous repair or scenario just to save your customer money

2 Risk a HUGE potential insurance claim god forbid anything go wrong.

3 Is it really worth the potential consequences to make a tiny profit on this job.

i would explain this to your customer and i am sure they will understand or make them sign a total disclaimer on your repair understanding and reading what they sign i bet they change their minds.

Not knocking you for doing the job but that next service outside has way more profit and a 1000 times less risk.

sean