What would be your reaction?

definitely i'll be angry with her or him. cause thats bypass of trust!

if best friend cheating me, i must ask her/his why she is doing so . i belive any problem should me solve with communication . if her answer satisfied me she is doing under some pressure thank o.k otherwise Tata Tata Bye Bye for ever . Once the person have a cheating habbit never being changed .

leave him and not to think of him again so as not to waste time....

i never gona talk to him again or may b give a chance

i would definitely feel bad. the frnd whom i trusted so much cheated me!!!! i will never ever maintain relation with him again.