Q: Brother in law changed oil in my 2007 mustang I JUST got. Oil light came on while driving. Car shut down. Engine light and battery

If all or most of the oil leaked out it is possible for the engine to be damaged or to seize. In that event, yes, the engine would have to be repaired. You can try turning the engine over by hand, using a long enough wrench and socket on the crankshaft pulley bolt, to see if it is seized. If the engine won’t turn by hand, it will have to be removed and repaired for sure. However, if the engine turns over by hand, replace the oil and try to start it. If it starts, measure the oil pressure and if good, and if the engine runs, you can probably use the car. If the engine makes noise, runs rough (possibly sticking valves, etc.), or has low oil pressure, you would have to repair it. As far as how this happened, although there is now oil everywhere, there will be a visible point of leak origin. Typically the cap on the valve cover comes off, or is left off, or the oil leaks out at the oil filter base. If you are interested in an engine replacement or rebuild, YourMechanic professionals in certain locales can assist with that. Please simply inquire based on your locale. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.