Why would a check engine light come on? My car does not seem to be having any problems? Is this standard?

There are a lot of reasons the light can come on and not affect the way the car runs. Your friend did nothing wrong in changing your oil. The light is probably on because of an emission or EVAP problem. Neither are a big deal unless you need to pass emissions test. However, get the code looked at just in case it could be something else. Autozone will check it for free.

thank you!