I worked at Radio Shack a long time ago when it existed. The scariest customers were mothers and fathers who waned to buy a phone tapping device. There were two models, one that attaches with a suction cup to an extension line to a tape recorder, the other plugs into the wall and attaches to a tape recorder.
I couldn’t not sell these awful phone tapping devices because it was my job but I’d try to dissuade in some way. I don’t know their situations of course but if your relationship has collapsed to that point, well, it’s sad isn’t it?
Conversation is another option you know.