I guess you need to wait till she calms down a bit. I would possibly suggest talking to your OH, maybe say he could talk to his DM about her financial situation, what accounts she has, bonds, etc. I'm guessing she has someone like a financial advisor with her financial situation. Maybe they could see the FA and get them in the loop, get the FA to check that nothing has drastically changed.
It's made me think of something else, if GS isn't the angel you all think he is, he has access to her accounts pretty much. You definitely need to get a check on things, get OH added as someone who can talk to the bank and FA. If possible get statements so OH can sift through them. Might be a tedious job, but do it backdated as far as you can.
If GS has stirred the pudding with this £10k situation, what else has he got from her? OK she doesn't have to justify herself, but at the same time GS shouldn't take the piss if he is. But it should be considered as a possibility.
If OH can keep an eye on things, plus the FA if she has one, so if anything does happen it can be sorted quickly.
I understand that she may be independent, but your Grandson has made her a target now.
I'm really sorry, hopefully it comes to nothing and everything is super. It's better to be on the safe side. I really hope I'm wrong about GS. Also consider valuables she has, maybe get them somewhere safe for now like a safety deposit box.