Betrayal? how do i deal wit a friend that sleeps wit ur boyfriend of 3 years rite under my nose,how do i ask ?

i hav a friend of 19 years and we been best buddies,i do anythng 4 her,til recently i found her number in my bfriend phne and confronted him bt he clearly denied knwing her of having her number, he left n 30mins lata she rang to ask me why we argued wit hm,and it al hit me they had communication bt i was stupid nt to see that.,how do i revenge her 4 betraying me that way n being dishonest