A good majority of trans people never do stuff like this, because this isn't necessarily a trans issue. However it does have a connection, but it's more of an innocent connection than anything.

Essentially when someone transitions, it's a super confusing time and there's a lot of mixed feelings about people and situations. Something I've seen with transgender people is that they don't know how to respond to the gender they've transitioned into, in this case she doesn't know how to respond to other girls very well. And while it's not necessarily what I'm trying to say it's more in terms of the fact that she's her best friend, and may be a bit envious or is possibly watching them have sex to understand certain interactions during sex while being female. I know it sounds outlandish but I've heard of this happening before but wasn't one of my friends.

One of my closest friends is non-binary, their sexuality changed completely when they came out as trans, a lot of their friendships with people changed as well. They found it incredibly difficult to judge certain situations, and especially had a hard time with men, considering that they transition female to non-binary.

Ultimately I'd wager this is more of a personal issue rather than a trans issue oh, but I definitely would be neglecting reality if I wasn't throwing into the equation.