What would you do if your bf cheated on you with a gir who smelled like fish?

idk if he will do it again . he treated me soo nice and cared for me he bought me anything iwanted but then he did this i hope you can hear me out on this . please help. and does it make a differene if you consider this girl one of your best friends ? ive known her for about 5 years . please help mee !