Multiplayer mined blocks not mined

My friend and I always play in the same world. He mines whilst I build. He asks me to come and look at an area he has mined out as he is suffocating in invisible blocks. Low and behold he still has varied blocks totalling to 3 stacks of sandstone scattered around the area he has apparently fully mined. We both shared photos and to no surprise we both see different things, he has nothing and I see blocks.