Make sure the ANGLE of the instrument is correct. Often, young bass clarinet players play with the instrument directly out in front of them, which with some (many) necks, especially on cheaper or older instruments, places the mouthpiece at a nearly flat saxophone embouchure angle.

Move the bell of the instrument under the front edge of the chair. To accomplish this, you may need your bass player to sit on the front edge of the chair and to lower the peg.

The mouthpiece should point "up" at about a 45-75 degree angle instead of being level with the ground. That makes a huge difference with bass clarinet tone quality and intonation. In other words - the bass clarinet mouthpiece should be facing nearly as high up as a soprano clarinet mouthpiece does, and you need to move things around to make that happen.