Once she asked... he was already in a sticky position.
If he had refused, its likely she'd have bad mouthed him and his business wherever she could, nasty people do not like being denied whatever it is they want.
If he accepts and then changes his mind, she'd do the same.
His business may be going well, and he may well not suffer from not accepting or even cancelling one job.
I think you are seriously under-estimating just HOW much damage one nasty, vindicative person (and it sounds like she really is that!) can do to a small business in their local area or even further afield, if they really want to.
He's done nothing wrong, she's a nasty cow and you are letting her get between you and your son, which is probably what she wants.
Pull up your big girl pants, tell your son you love him and this doesn't matter AT ALL, apologise for showing him the messages and getting him further embroiled in it and let it go.
That is how to deal with this - it would piss her off no end to discover that you don't give a FUCK.