Is it ok for my boyfriend to put his arm around his best friend thats a girl and walk around like that?

ive been datein my boyfriend for bout a year and a half and i saw him with his bestfriend that a girl walkin around with his arm around her and i got mad and he thinks its ok cause there just friends but i dont cause he puts his arm around me like that and i dont think its right for him to do that and i really dont know the girl to well he says they're just friends and its not bad i just want some opinons