That's tough man I feel for you! I'm always happy to give people feedback about their playing, but this sounds really bad and i wouldn't want to risk upsetting or offending them. Honesty is always the best policy though.
I would probably take a deep breath, say the recording didnt quite work out, ask them to come round and take a listen, and see what they want you to do with the recordings. They'll probably prefer to hear it themselves than to just be told 'dude your show sucked that night'. Maybe even ask if they want you do do another night in future if you want, giving them a chance to tighten up. Who knows it might be really good for them and they'll end up very grateful
Good luck mate!