They are usually held on by little clips you can buy from auto parts store. I would glue it with some kind of adhesive that is waterproof and stays soft like a sika product or similar. Those coming off are a known issue for Subaru

This answer here. Look at the other side. Do the same thing. You can also search her model Subaru and the year number to see what that part is called then look at YouTube on how to replace it. Even if you can’t find a video for that exact car it should give you an idea of where to start.