A friend currently living with his Mom would like to get away but does not know what's considered neglect. Any advice?

Whether that would be neglect or not depends upon how much help or attention the mother needs. If she needs constant supervision, then this would be neglect. If not, she may be just fine.

If she's doing well enough to be left alone, she should likely be told they are going out (she'll know anyway, so she'd likely worry less). If she is so ill that she wouldn't know the difference, they most likely should find a substitute caregiver.

Their need for privacy is completely understandable, but they've taken on caregiving and now need to responsibly make time for themselves. That may mean hiring an in-home caregiver for when they want to be gone.

Take care,
Carol

How about a hotel room? Wouldn't that be easier than trying to maintain another residence? If they got an apartment and had unexplained absences I would think they are dumping their support.

"Of course, they will not let her know of their comings or goings."
That is the red flag. That is what makes the difference between caregiving and freeloading.

Or, if they just want time during the day, how about activities may a senior center, or day care, if appropriate, for Mom? She would probably enjoy making friends and the activities that are offered.