I think it’s a bad idea. It sets the standard that your house is her house, which it isn’t. Furthermore, I believe there are legal issues if she stays there long enough (e.g. not being able to evict her very quickly if you want to). Furthermore, it disincentivizes your son from finding his own place when he’s old enough.
Also, your son’s girlfriend should not be spending the majority of nights over at your house at her age. I would encourage them to find income and their own housing if they’re that serious about living together.
🤷♂️ I just Googled it. Glad I could help though.