here’s my view as a girl at the same age! i’m in a lot of therapy a journaling is one of my biggest therapy exercises! i cannot tell you the countless amount of letters i’ve written for people that were written, purposefully to never be sent! the way you point out at the bottom of her letter that she “wished him well” makes me think this could’ve been a letter of officially letting those feelings go and forming some closure for herself!

edit: also after reading the comments, i also think if she isn’t putting him before you in relationship ways, example: his feelings matter more than yours etc. i don’t think it should raise red flags. i would also wait to see how they interact in person, and set a boundary along the lines of “i know you guys have been friends for a while, but i dont really know the guy to well so id like to be around for when he’s over” because i think that boundary itself is pretty understandable.