That makes me so mad. Unfortunately the idea that guinea pigs are disposable pets that don't deserve medical care is really common where I am.

I once had someone suggest I put an ill piggie in the freezer as a kind of free home euthanasia and they were dead serious about it! They owned rabbits and I was too scared to ask if that was actually something they had done in the past because I was horrified at what the answer would be!
I would make sure you tell them that you and your son really enjoyed having the piggies, and that if they ever decide they don't want them, you would be happy to take them from her (even if they are sick and need medical care.) Maybe she will take you up on the offer. Short of involving authorities (depending on the laws where you live), which might be a very obvious tip-off to her who reported, I'm not quite sure what your options are. Poor little pigs.

Why get pets if you aren't willing to take care of them?