If crucified for a period of corporeal punishment--let's say 6 hours tied naked to a cross for shoplifting--that girl could be further humiliated by having to pee while exposed publicly on a cross. I'm sure a female awaiting corporeal crucifixion would have been instructed (or, if in jail, given the opportunity and means) to clean out her bowels the day before to prevent any accidents while on the cross as well as instructed to to empty her bladder before punishment commences. Even so, she'd probably still piss from the stress of her punishment and being given water to drink while hanging to keep her hydrated.
If crucified as a death sentence, and nailed to a cross, loss of bodily functions would be the last of her concerns, as others have pointed out.
If public crucifixion as accepted in a society then I don't think there'd be any problem with seeing someone peeing or even pooping from the cross. Using a catheter and bag would just draw attention to her need to pee and just cause her more humiliation. (Though there is the kink factor to be considered.)