How to I not lose my best friend to someone?!?!?!?

My best friend and I were SUPER SUPER SUPER close ask anyone kid and they would say there was no one closer. but the thing is that we are of opposite gender.. i'm a girl and he is a guy.. DUH. so i have a friend that i made that this camp. So me and her are older (i'm 15 she is 15 and he is 13) so we were old enough to be volunteer in this camp and he wasn't so my best friend and i were hoping i'd be in his group but instead she got put with him. So this past weekend they spent about 15 hours together in total. and now they have been texting, snapchatting, kiking a lot and I haven't talked to him for more than 5 min... should I worry or is it okay? tell me what to do?!!!! SERIOUS ANSWERS ONLY. NOT A JOKE