Why is there gas in my oil?

My friend bought new, a craftsmen lawn mower with a 6.25 ft-lbs torgue series 190cc briggs and straton motor two years ago. He did not have any problems with it until this spring after he changed the oil for the first time. He ran it only once and it was having problems like blowing alot of smoke and running really rough and there is now a bit of oil that is on the deck of the mower around the motor. He didint want to deal with it so he gave it to me and got a new one. When I got it home I checked the oil and it was way full and smelled a little like gas. I dont know if he just put to much oil in it and thats why it is having problems or what. So what do I need to do to get it running again.