actually i probably would say something.
i mean saying the words 'thank you' costs nothing, i am gobsmacked that it didn't just come out of her mouth.
im not suggesting you go round kicking off, but suggest that she should thank your son, you could say that because they have not thanked either of you for all the time and effort you put in, that your son was a bit worried he'd done something wrong, and you are concerned he will feel disheartened and unnappreciated and put him off helping others again.
i also would not do it again now anyway, even if she does thank you - though if you say nothing now, next time she asks and you say no, if she asks why you will have to say 'because you didnt thank me last time' - which will seem petty... or make some excuse.
the dog wont suffer they will just get someone else to do it.