being a TA is a great way to get back into teaching.
mother of child in DS' class was ex teacher and when her dd starte d school she got a TA job to get back into the classroom environment after being out of it for 7 years. When a teaching job came up at the school she applied and got it - the TA job she did for a year was a good way to ease her back in but without throwing her into the deep end, until she was ready to go back into teaching.
I am on the PTA, I am a governor and I help out wherever possible. When the TA has been off sick I have gone in to help not because I was asked but because I was aware that the TA was off sick and the teacher was therefore under more pressure and so have offered to come in and the teacher has been very grateful.
This parent in question might have done the same. Or if she's there regularly she may be the one the teacher thought to ask because she knows her and has a good rapport with her and the children know her.
If you want to help out then you have to be pro-active ime. Often teachers won't ask because it is volunteers they need, and asking is like admitting they need help and that's not something a lot of teachers want to do.
So approach the teacher after school and say "I'm available all next week and I was wondering if I could come in for a morning and help out in any way? Maybe... oh I don't know... thursday? or whichever morning is best for you".
The ones that do annoy me are the ones that go in and then walk around the playground pretending they are some sort of authority in the school and know everything, when in fact they rarely know anything.