I don’t mind a bit of construction work. Wouldn’t be hard to tear down the drywall in the bathroom and pack the stud walls with Rockwool soundproofing insulation. And also swap the door out for a double-thickness solid core sound-deadening door with a sealing sweep. That’s like a couple weekend’s worth of work max.
Also she is an absolute saint and would probably be fine dealing with it to stay with me.