I had to cancel on my friend tonight. I felt bad when I did too
That happens sometimes. Sometimes life just gets in the way. Heck, sometimes you just decide you can't physically do a night out, and that's okay, too. I'm sure they'd understand no matter what. Maybe you can make it up to them somehow.
I hope I can too
Maybe take a few days to think it over, and then when the weekend comes, think of a way to surprise them in order to make it up to them. Or if nothing else, just be as honest as possible and not try to cover anything up when you need to bail on them. They'll appreciate that the most.