Argh I need an outside perspective, so cross about this.
Dd works varying shifts Monday to Friday.
There was a special weekend event so she worked Saturday. She doesn't normally work Saturdays and didn't want to but did! 8 hours in the hot sun without a fuss.
Dd saw boss today who said she hadn't been rotated on for Saturday and must have rotated herself on therefore she wouldn't get paid. 
I have said she needs to make a fuss about this. The boss was there Saturday and not once did they say to person A 'you shouldn't be here' or similar. She didn't want to be there and wouldn't have been if she didn't think she had to!
It's a private organisation so no-one higher that she can complain to.
AIBU?