Best friend joke

Mark was passing by the bar on the way home from  work when he sees his good friend Tom gulping down one shot after another.
Fearing the worst, Mark charged into the bar and confronted Tom.
“Tom what’s going on?” Mark asked.
“It’s my wife Beckie,” Tom replied. “She ran off with my best friend!”
“Hey wait a second! Said Mark “Aren’t I your best friend?”
“Not any more,” Tom said with a happy smile.
“He is!”