Horrible idea. My sister let a very close friend stay in her basement out of the kindness of her heart. She was in the middle of a divorce and I guess desperate as fuck. But within 4 months her adult daughter had moved in and about 3 months later her "ex"-husband had moved into the basement too. It took almost a year before she could get all 3 of them out.
My sister is a neat freak and I heard horror stories of her cleaning the bathroom after they left and finding shit like half eating slices of months old pizza in cabinets, under the bed, etc. Not to mention having to junk a bunch of shit in the basement kitchen like the microwave and tear up the carpet and cheap pergo flooring and replace it all.
I was even unintentionally involved in getting them out because I was called up to help move them out. That was not fun packing up their junk (and it was all 100% worthless junk) and then driving a uhaul 90 miles to a tiny subsidized apartment and carry what seemed like millions of heavy boxes of what might as well been literal shit into it.
I hated that day, but I was glad to be a bit of help in finally ridding my sister of her poor decision to take in a friend in need.