An oil change won't affect the oxygen sensors -- unless the oil that they put in is heavily contaminated -- totally unlikely. As for what is causing the problem, I am afraid that we don't know what problem you are talking about. There is a long list of oxygen sensor related trouble codes; some may lead to a bad sensor, others to something entirely different that affects the sensor output. I am assuming that the check engine light was on, and someone found an oxygen sensor related code. Post the code here, and you may get some tips. Honestly, I don't see much purpose though, if she is going to take it to the dealer. For sure the dealer won't need this group's help in diagnosing the problem...
IK