help quick decisi

so I was with my best friends (guy) girlfriend and I was hanging out with her , 2 other girls and another boy. my best friend does not like the boy that was with us and doesnt like when she hangs out with him and she knows that. all of the girls are lying to him when he asked"no one else just u guys were there" and I feel terrible for not telling him the truth because hes my best friend. but if I do he might tell his gf(not likely) and maybe slip my name but we are good friends, idk if I should tell him or not, because its a biflg deal to him if she hung out with him because they used to have a thing. what do u guys think?? comment if not what to do please