When flirting ends up with exchanging phone numbers is it cheating yes or no and why?

a few years back my then very trusted boyfriend was out with the guys 1his best friend and 2 by my best friends husband.Well they were there sitting chating away with another female. My boyfriend was supposedly flirting up a storm and acting unattached with this random girl and at the end of the night they exchanged phone numbers. I've heard all this recently from my bff who heard it from her hubby. So who do I believe one is supposed to be the love of my life and the other a committed man and has proven it. Serious answers only I know how easy it is to give lame one's. And yes I have confronted him and he's denied it and his best friend has backed him up. But this little comment was made what happens at a derby stays at a derby.