Maybe see if the friend taking care of your snakes is willing to do some rats... make some travel bin cages and pick your best females / males and put them in bin cages and take to the friend's house? That would be the only thing I can think of. My rats go through at least half a large water bottle a day... More in the heat (like 1 large water bottle a day) so I change water daily, and that is with only 3 adult rats per bottle.

Maybe offer to compensate the friend more if he is willing to watch the rats and bring your own food /bedding / etc. for him to use while you are gone.