I totally believe juice can be the reason for loose poops but how much juice is she getting for it to be this bad in just one day??

I would just keep telling the mom every day how many poops she had (altho, I'm sure sending home all the dirty laundry would be the first clue lol) and keep stressing to mom how you haven't given her any juice at daycare. Hopefully mom picks up on this.

I would also be telling mom that until her poops become more solid or she starts telling you when she needs to go that you will require her in diapers for sanitary reasons.

I also would be calling for pick up if she has more than 2 or 3 loose poops (can't remember what you said was your policy). This should hopefully prompt mom to take the juice thing seriously and quit giving her so much!