With a dog: Dogs get seperation anxiety often when someone plays with them etc. etc. then makes a big “goodbye’ fuss and leaves (basically a rewarding experience (play) followed by negative punishment (master leaving) and this results in serpation anxiety.
I’ve never heard of it with a bunny, (to be honest thats cute!!) but I’d go about fixing it the same way.
Your daughter needs to drop any ‘leaving’ or ‘goodbye’ routines completely-no even saying ‘goodbye’; Also she shouldn’t hold him and cuddle him the whole time she’s home at all.
She should also start doing a series of ‘practice goodbyes’ where she doesn’t say goodbye, calmly leaves and then as soon as he’s shown a hint of settling (even if he’s just taking a drink) she can come back in and cuddle him. By gradually shaping his behavior to not be frantic when she’s gone, and not giving a hint of when she will be gone, and not leaving at the end of a cuddle session this shoudl clear up the behavior.
I’d say be very patient and allow about a month of this, and see where your at!!