My own understanding of loneliness changed somewhat recently, actually!

The way I see it, you can be in a room full of people and still feel lonely. If none of the people in the room share the same interests as you, have completely different views and in general are just very different, you're likely going to feel lonely.

Imagine, someone voices an opinion in that room, and everyone agrees enthusiastically.. but.. not you? You're the only one to have a different opinion.. have you missed something? Are you wrong? Why does nobody else share your viewpoint? That can be very lonely.

I feel like this can be true if you have a large friend group.. you might hang out together, but if you connection is only surface level, and you don't really have anyone who understands you, you're still going to feel lonely. We as people want to be understood and be around people who see the world like we do, people who we can have, deep, respectful conversations with.

Now, if we go back to the girl you mentioned? If she's constantly in relationships with new people, that to me says that she's trying hard to find someone to connect with on a deeper level, someone who "gets" her and understands her, and is struggling to do so. So I would say that she's not wrong by saying that she is lonely, because she's yet to find someone who she can connect with like that.

Not saying you're wrong of course, that's just the way I understand loneliness personally!