Do you really have to do it text? It's not good and may impact on friendship, references etc and might end up with her calling you so you have to speak to her anyway.
If she is your friend, I'd say you owe it to her to speak to her and tell her why. This is her livelyhood.
If all this was going on in my pub I'd be furious and really upset if I found out a friend didn't tell me.
It soubds like a dive and the staff onviously don't care. Is she likely to be happy about that?
If you really do want to send her a text say something like "Hi x. I'm sorry to do this by text but I am handing in my notice. Unfortunately, the job isn't working out as we planned. It looks like the hours we originally discussed aren't suitable and I can't be available when you obviously need me. If I'm honest, I also wasn't expecting the level of cleaning I would need to do to, for instance I didn't realise I would have to clean up so much mess in the toilets. I'll work until x date so you can get someone to replace me and if you need me to, I'll do a couple of shifts with my replacement so they can learn the ropes. Fuckmyuterus".
Or if you want to be more honest:
"Hi x. I'm sorry to do this by text but I am handing in my notice. Unfortunately, the job isn't working out as we planned. We originally discussed the hours I was able to work but it's obvious they don't align with what you need. If I'm honest, I also wasn't expecting to have to clean up bloody tissues, mess on the toilet walls or so much sick. I'm concerned that your staff are not acting as they should be eg drinking after work or staying the night. I've also found white powder on the tables. My last shift will be x which I understand is short notice but should allow you to find a replacement. Fuckmyuterous".
Or, if you really want to chicken out:
"Hi x. I'm sorry to do this by text but I'm handing in my notice. It is obvious that you need me to work hours I just can't do. I also wasn't expecting the amount of cleaning there is in a pub! My last shift will be x to allow you to find a replacement. If you need me to, I can work a coupke of shifts with them to make sure they know the ropes. Fuckmyuuerous."
As I say though, she's meant to be your friend.