Girlfriend has a bestfriend and they seem to close should i worry?

ok so my girlfriend has a bestfriend the have known each other for about a year ima bit worried because he goes over to her house and she mentions him when im around her alot and he considers her a bestie but he also has a girlfriend but they just seem a little to close to me he also sends her hearts and teases her like the way i do and to be honest it gets me realll madd should i worry or should i not btw im 15 years old so dont juge lol well thank you and id really appreciate and answer(: