2001 Ford Mustang Rear Main Engine Leak

My 16 yr old son took his car in for oil change. It came back with an oil leak as noticed in our driveway. We took it back and they said it was caused by the rear main engine oil seal and that there was evidence of oil on the bell housing on transmission so it must have been leaking oil already and that these things just go. We took the car in for oil change because the service light came on and I thought it just needed an oil change. It was not leaking oil, my driveway was clean. My questions: #1- is this possible? #2-what is it going to take to fix it? I need to know what questions to ask or what to expect to be sure I authorize the right fix. (I am single parenting female who knows nothing about these things, can you help with some advice?) Thanks!