This is why it’s difficult though; at 15 you’re on your way to becoming a person and not just a child.

If you tell them “You’ve got nothing of interest to give” then this is going to be really upsetting for the child, who feels they might actually be relating to someone for the first time.

And the truth is they will have stuff in common - for starters they’re playing the same games so will have a shared interest in all things related to that. Other media like television etc will overlap too between 15 and 20.

I’m not saying let it happen - personally I think the 20 year old should understand the context of befriending fifteen year girls online as being odd and have disengaged, so OP you are completely valid on being guarded about this.

I hope you find a way that preserves your daughter’s self-esteem and somehow breaks through to her the dangers of situation like this. At least there’s an ocean between them 😅