You only need a few volts to jump start your fren's car. You are
not powering his ipod, radio, air-con, etc.....
Once his car is started, his car engine will take over to power
his car and you can go home to sleep.
Well, if you are worried that your battery has dip by that few
volt, keep your engine on or drive around for a few min to let your
alternator charge.
Usually, when the batt goes flat, it's either owner forgot to
switch electical appliances off e.g air-con, lights, harzards,
radio, etc...; or turned on such appliances on without the engine;
or the batt is near it's lifespan; or that the alternator is
defective.
Check his habits and send the car in for a routine checkup.
I'm a layman but I think Playmemory or other pros may be able to
advise you more on this subject.
Good luck.