I have a background working with young people/ safeguarding.
From the info you've given it doesn't sound like a love interest type situation where the older one might be taking advantage. That would be the situation where I might be worried and that is the reason some people might be more worried if it was a male.
From your posts, it sounds like they have things in common and have hit it off. It's the whole group she's hanging around with and not just your daughter. It doesn't sound like one young person is being singled out.
From a safeguarding perspective, I wouldn't be too worried about this as it doesn't sound like there is any malicious intent.
But I'd try and keep up with your daughter generally, let her be open/ chat to you about whatever's going on, so you get a general sense of this person.
Sometimes, sadly, there are criminals who will send someone close to young people's age to develop a relationship with them, and online gaming is a common way for them to find vulnerable young people.
So with that in mine I'd want to keep a 'watchful eye' but I wouldn't be worrying as such, because nothing untoward has hapened.
Is there any way you can meet this person if you haven't already? That might give you a better sense of what they are like.
Also, is your daughter generally mature and sensible or is she a bit vulnerable/ young for her age? Do you think she could be at risk of being swept up in something and being a bit naive?