Question without notice. If an aircraft flat battery is discovered, is it possible to undertake a "jump Start" from a car if one is handy, rather than taking the risk of becoming a one-armed wall paper hangar?
Of course it's possible. But...
Jump starting is not risk free either. You had better know what the aircraft's battery voltage is, what terminal is connected to earth (airframe) and what jumper lead goes where and in what order. And as AB notes, if you have to have e.g. a cowl panel open or removed and the cover removed from the battery box, what's the plan to get them back on/closed? Note also that there's going to be a fair bit of wind and vibration - could be
very bad if a jumper lead shakes/blows loose and touches the wrong thing in the engine bay ...
(Note too that some aircraft have auxiliary power sockets. A much better solution, if available. The POH will explain the correct procedure for using it.)