I think you should explain that they are good friends, but not bf/gf and why. Tell her that to have a boyfriend that means to be "dating" and you have to be able to go on dates - drive a vehicle, earn money to pay for your entertainment, get good grades in school to earn your fun times etc.
My daughter calls my best friends son her "boyfriend." She only sees him a few times a year when my friend and I get together with the kids, but I always remind her that he is a boy who is her friend.
It's innocent and and they probably get a lot of support from each other as friends - and probably even lots of attention (and dare I even say admiration...yikes...) from other members of the class to have such a close friend of the opposite sex.
I think it's great that you and his mother are in contact and you should continue this. Maybe even get together with the kids so you can both see how they are together - innocent or too touchy/feely.