D.B.
Find out if the medical center/office where he had the study also has a "mask clinic". Many of them do, and patients can come in for free to meet with someone on the neurologist's staff (nurse, PA, etc.) who can help with these issues and do some "tweaking" with the mask. It's possible it's not the right one for him, but he should take it with him and let them work with him before starting over with a new one.
Sometimes the respiratory company that supplies the equipment has well-trained staff. Did anyone help him with this, or did they just ship the equipment to him for him to fend for himself? That's not how it should be.
Either of the above should be able to help him with fitting the mask to his face, adjusting the straps, angling the piece that rests on his forehead, etc.
Usually there is a short hose that connects the mask to the longer hose that goes to the machine. That's for a quick disconnect without taking the mask off entirely, for exactly the purpose you describe - a trip to the bathroom.
Encourage him to seek out help with this - absolutely, REM sleep is critical. It does get better with repeated use, but it's important to get it fitted properly.
Also, be sure he got the proper instructions for washing it and for reassembling it. You don't want bacteria building up . Usually once a week is fine.
On the plus side, if the room gets too light early in the morning, people with BIPAP and CPAP machines can just throw the covers or a pillow over their heads, and still be able to breathe!
Good luck!