I feel for the OP as well. Whilst the discussion about being a TA either voluntarily or paid as a way either back into teaching or to get a good buzz from being back in the classroom without actually having the hassle of returning to qualified teaching again is interesting, it's perhaps a different issue to what the OP has raised.
Whilst the mother in question may WELL be a qualified TA/teacher- and it's for the school to publicise this fact- she may equally as well not be. Many of us will groan at reading the semi-literate entries in our DC's reading log written by a well meaning mum-who-helps (Q: "TeslagirlDS done really good in his reading to-day"- seriously). NB I didn't go in complaining!
Whilst the woman's motives MAY be pure there is no doubt about it that SEVERAL mums who do this ARE on a bit of a power trip. Hints about 'how much THEY'RE party to, 'if only you KNEW what I know', the 'ah, you may not know ALL the facts (but I do!)'. Seen all that! Also, there's no doubt about it, every dynamic group like PTAs, governors, parent-helpers NEED to encourage and bring on new blood so that the DCs who follow have the benefit of nurtured experience. You don't get this when one person or small group does dominate that arena.It DOES discourage others and perhaps stifles innovation. The OP has made it clear she has offered her support but it appears to have been rejected in favour of Mrs Do-It-All.
Needless to say, our school has their share of these but is sufficiently large (inf and juns) that they NEED a critical mass of willing volunteers to keep it all afloat. For 3 years the 2 PTAs were dominated by one set of parents- chairing, governing and, well, controlling. Didn't bother me as I felt they were doing a reasonable job but I knew for a fact several parents' help had been rejected outright THEN this pair had the gall to complain that they were left to do it all! Incidentally they've now moved on to chairing the local junior football club, scouts committee and I hear secondary PTA! My observation is they seem incapable of doing anything if they can't be in charge. Once again, I reiterate- best of luck to them, never caused ME any personal grief but I'm quite glad my DSs sporting club lives aren't dominated by them since they play on another team and are in a different school.
Finally it's a reason I rejected a small local primary of 40 DCs- the discovery that the chairman of the Board of Governors (who was a bloke I was at that school with 40 years ago!) was also chairman of the PTA and that his wife was a full-time parent helper and his mum was school receptionist/secretary. No thanks!