I don't know how old your student is, but I worked with a teenager for a while that rode at a crappy hunter barn and pitched herself way too far forward. I was having trouble to help her to return her pelvis to a balanced position. Finally, she blurted out in the middle of the lesson "Oh, I get it, you want me on my buns and off the taco!". After that, it was a joke to "crunch the taco", but she did find a seat.
I would put her on a lunge line, without reins and do a lot of work sitting and a lot of transitions. It's impossible to ride a sitting trot or a correct sitting canter from the tipped forward position
I would also show her a human skeleton, and show her what happens to the rest of her body mechanics when she tips forward. I'd then have her locate her own seat bones (sounds goofy, I know, but people don't really know where they are, and in results in a lot of confusion about position.) Once she actually knows where they are and can sit in the saddle with seatbones pointing straight down, have her rock forward and back in the saddle repeatedly and then find the correct position again. She needs to correct her muscle memory so that tipped forward no longer seems correct.
I have horrible forward tilt muscle memory. Every so often, I lift my legs completely up and away from the saddle so I have only my seat bones and butt to balance on. If your student learns by feel, that might help.
In classical dressage, one doesn't sit back "on one's pockets", & the deep seat takes much riding time to develop, because the anatomy needs to shift about/stretch some, over time. The small of the back should have a slight hollow, not be rounded. One should be as though standing, albeit a wide-legged stance, to accommodate the horse.