Most kids aren't potty trained at the age she is now. So she's far far ahead of a lot of kids.
Kids don't "potty train" during the night time. When she starts staying dry every morning for weeks and weeks....that's when she can wear underwear.
You're the mom, tell her no, she has to wear a pull up during the night time because her body isn't ready to stop making urine during the night.
Her kidney's are supposed to sort of turn off when she goes to sleep. If she continues to make urine during those sleep hours she has no ability to stay dry. It's a chemical action by her brain, not by anything she does or does not do.
Drinking fluids has very little to do with being dry or wet in the morning. If her kidney's stop producing urine she'll be dry no matter what she drinks. Being dehydrated makes her MORE likely to wet during the night.
When we wake up most of us have to go straight to the bathroom to urinate. That's because our kidneys just turned on and made then filled our bladder as we were waking up. Some kids are so out of it right when they wake up they don't go straight to the bathroom. That's when they wet their pull up.
If she's soaked that means she's still going all night. She's just not physically ready and there's not much you can do to make her brain produce the chemical that shuts the kidney's down while she's asleep.