Boyfriend cheated?

Genesis
Ladies I have a problem. My boyfriend of almost 2 & 1/2 years just confessed to me that almost a year and a half ago, a girl we used to work with grabbed his penis (over the pants). She was stroking it for like "3minutes" but he claims he never got hard and it was more of an awkward situation. He says nothing happened after that. I can testify that they stopped talking at work but I didn't know why. Should I be upset over something that happened so long ago? Is it considered cheating? Please help. :/