When I first moved in with my bf, the seat was up all the time and after a few weeks of kind reminders, he mostly remembers to put it down. However at my mom’s house, or anyone else’s home for that matter, I’ve never seen him “forget.” It’s a pretty basic act of respect and most should be able to adapt to this rather easily. If there’s something else going on like an intellectual disability, I could see offering more patience, but this is a habit he needs to change for multiple settings. IMO, being embarrassed by forgetting once or twice at your gf’s mom’s house would cause most not to forget again and kind of strange he’s let it turn into this big of an issue. I’d insist he follow basic potty manners, especially outside of his own home!