That's odd about the 500e only having four speakers! It does seem like it should have rear speakers. Whether you can add them depends on a few things. Sometimes, cars are pre-wired for rear speakers even if they aren't installed in certain trims, which makes adding them relatively easy. Other times, you'd need to run new wiring, which is more complicated.
Check if the car's head unit has the necessary outputs and settings to support rear speakers. Your friend could consult a car audio specialist or let's see if someone might have already done this upgrade and can offer advice.