Putting out a bad product/untight band

If they had just tabbed the shit out in GuitarPro, you wouldn't have this problem. I always tell bands to tab it out in GuitarPro, so I'm not flying blind trying to assemble tempo maps in the DAW. Just make it a MIDI file, plop, there I go. And we even have a nice (in-tempo!) file to record to!
 
Something you may wanna do in the future is, ask the band if you can watch them at one of their practice sessions so you can get an idea what you're working with. Then after their practice if you can hang out long enough, have them take you out to lunch or dinner (Their treat) and talk with them about what they need to do or if you're interested or not in recording them. I do this

I agree with this!

plus if the band is untight, it's not going to make you look professional as an engineer. So just be careful who you entwine your name with.