I caught my best friends GF smooching a stranger , shd i tell him abt this ?

My best friend and his GF r going around for 4 years now and their marriage is fixed for next , lately ma frd told me dat his gf is behaving a little strange and hav changed a bit , and jst a week later i saw his gf smooching a another guy at a park . i was shoked . My frnd loves this girl very much he spent money like crazy buy her gifts , dinner dates , jewellery ect . he once told tht he cant imagine his life without her and cant marry ne1 else except her . Now if i tell him will he trust me ..... how do tell him ... should i speak to his gf and tell her wht i had seen , or jst keep shut and wait , coz this gal is cheatin on him & i know abt it . wht if she completely denies tht she cheated on my frnd and says tht i was tryin to spoil her name and stuff , or tht i am tryin to misguide him coz i was jealous . ... i know his gf and she is totally capable of doin so . ................. i am not scard of any one .... but st tht i dont want 2 lose ma friend