Elections need to be conducted in a way so that no accusations of fraud could reasonably be made. This sounds like a question of where to draw the line.
If someone returns a ballot for their spouse and maybe another close family member, that’s probably okay, but it’s also possible that they were all filled out by the same person without the approval of their spouse or other close family member.
If someone pulls up in a van with a suitcase full of ballots, that just looks really bad. It’s possible that it’s entirely legit, but there’s a much higher chance of fraud.
If someone wearing sunglasses at 2:00 in the morning stuffs a drop box, it’s possible that those were all ballots filled out by different legal voters, but allowing that’s just asking for accusations of voter fraud.