If MIL had capacity to give an LPA to the son and decided no one is to be notified, that is fine.
What is more concerning is that she is "being told what she can and cannot spend her money on". If she has capacity, frankly she can do what she likes. If she wants to go and buy a porsche and live it up at the Ritz, if she has the capacity to do so that is fine! If he disagrees, she can choose to revoke the LPA (if she has capacity).
The point of an LPA is that decisions must only be made in the donor's best interests. However, insofar as possible the donor should be consulted and encouraged to make the decision herself. It is not for the Attorney to "control" the donor. There is a fine line between acting in her best interests, helping her with her finances and removing her personal autonomy.
If she is in the early stages of dementia, I imagine she has the ability to make many decision but probably struggles with day to day banking and stuff - particularly now so much is done online.
Eg, my mother and I are attorneys for my grandmother. Mum deals with her every day banking and regular payments. However, Nanna has capacity in most areas. So if Nanna says "I want to take the Smith family out for lunch they were good to me and Grandad", Mum and i say, "excellent, we'll make that happen, where and when would you like to go and we'll draw the cash out for you". If she says "I really like that xxxx mattress, I want one", we'll talk to her about it and make sure it's right and then we'll order it. It's her money, it's her decision. She has more blummin cardigans than you can shake a stick at but she constantly wants new ones. Fine. Her decision. However, if she said "a young man called me from xxx company and he says if I invest £5000, when I die you'll get £50000 and I'd like to do it. He was such a nice young man and said Grandad would want it", we'd be saying "Lets investigate that and talk about it a little"..............
The role of an attorney is to facilitate - not control. Accepted someone with no capacity needs someone to control but MIL should be allowed to partake in decisions for as long as she is able.