Is this wrong? or should i let them be...?

im bi and my best Friend who protested this for 2 years finally gave in and actually began to try it out herself to make a long story short, and supirsingly she met this girl and they have been dating for a while i just met her and shes 15... my friend lissa is 17 and she will be 18 may 25th the girl will be 16 may 2nd i dont know if that's ''okay'' you tell me is she too young for my bestie? they are really 'into each other' and i dont wanna ruin her experience but i dont want this to ruin her life either i mean sh acts immature around me but lissa says in private the girl is very mature...too young or am i over exaggerating?