I typically give people the benefit of the doubt and see it as someone who is just trying to find some help and made a slight mistake in judgement to email parents. I don't think it was the most professional thing to do, but doubt she meant any harm by it.
I would guess that if any parents were in the plumbing business, they would be happy for the chance to pick up some business. I don't see it as her wanting something for free. Maybe I'm not as jaded as others, but I've never had anyone ask for help like that and not expect to pay. Besides, that should all be worked out before the help was provided and if that was the case the "helper" could let everyone else know what this person's game was (but my gut reaction to that email wasn't that she was looking for a handout.)
If I got that email, I wouldn't have been bothered by it at all. If I could help, great. If not, just delete it and move on. What would bother me, is if this got to be an on-going thing and she was asking for help. At that point, I would say something to her.