Sabaki

Make sure you right click the block first with chisel in hand. You'll know it's chisel ready when the hitbox changes from the whole block to individual voxels.

Use "/worldconfig microblockChiseling                                      [off|stonewood|all]" to enable/disable.

Sounds like he might be in immersive mouse mode? (Under Esc menu/Settings/Interface) In which case, holding <alt> will free the mouse, or you can just disable immersive mode. Hope that helps!