There is no reason that your friend can't use a charge line to the Mini to keep the battery charged. It should be a protected charge line, however. That is, it has fuses or circuit breakers to protect both the car and the MH. I use a Toad Charge system with my Honda Fit (which also uses an RVi2 brake). The Toad Charge has circuit breakers and a charge regulator that only charges the car when enough voltage (14v) is present to do so.
If you have to leave the key in the ignition to keep the steering wheel unlocked, then using something like a Toad Charge works well. You do need to shut off the fan and radio and anything else that you can.
Based on your description, it also sounds like your friends battery is ready to be replaced as well.
