My husband's mother has tried to kill herself a few times, most recently about a month ago and she ended up in a mental hospital (again). Her husband passed away about 4 years ago and she has been in a downward spiral ever since. The money she received from the life insurance policy was supposed to last her well into retirement; it has not due to her frivolous spending. Recently she had a boyfriend turned fiance who we determined (and she admitted) was only trying to marry her for what remains of her assets and was going to write my husband and brother-in-law out of any inheritance they would receive upon her death. They broke up a few weeks ago, but she started engaging with this man again. She put him down as her emergency point-of-contact when she was admitted to the mental hospital, so he knows about her suicide attempt and we are afraid he's going to try to take POA from her and spend her money as he chooses. She sold her house, which closes on June 1, to originally move in with this guy, but after they split she is now moving in with her sister (who is also a bit of a train wreck). We want to assume POA/guardianship before this man gets the idea to do this and set up a trust that will make sure she is able to protect herself and her assets in the future.
How would we go about doing this? How can we prove to a court she is mentally unfit to take care of herself and make good financial (and relationship) decisions when we do not have access to her medical files or her doctors?