Are you saying that the branches look dead or that the leaves shrivelled and looked horrible and like the tree was diseased before they fell off?
I ask because some acers (especially brilliantissimum), need to be planted in exactly the right spot and conditions to get good autumn colour. I have had three of these trees now in various gardens and all are horrible before their leaves drop. There is no autumn colour, they go a sludgy brown, shrivel and then drop off.
The first tree it happened to, I thought the tree had some dreadful disease as it looks so ugly. I have since heard the question being discussed on a recording of GQT I attended. I had not realised they were so fussy about location. The biggest thing I think they said was some sort of wind burn. I wish that I could remember the explanation now. I do know that ultimately it was not anything serious, it just looks ugly.
It makes up for its ugly autumn by being beautiful for the rest of the year!
The thing I am not sure about with your partner's, step mum's tree is that it sounds as if it has been okay in the past, or maybe it has not been that brilliant as you don't mention it looking spectacular before the leaves drop. I have three more acers in my garden of different varieties and all of them are absolutely stunning in the autumn. So, it might be that the tree is just not in the best location for it.
If it was my tree I certainly wouldn't do anything drastic to it until I had seen what happened to it in the spring.
They can also get diseases though, many years ago, I lost a potted one in the autumn, the leaves did the same thing but when I looked closely at the tree, under the bark it appeared to have growths all over it. This was before the internet was a mine of readily available information, so I have no idea what it was, but it might be worth having a good close up look at the bark.