Setting the scene
A woman with 2 male cats moves in with her boyfriend. The cats have always got along very well. They are both healthy and the litter tray is well managed. There had been no problems with peeing outside the litter box before she moved in with her boyfriend. The older cat particularly likes the boyfriend.
The Slate.com answer (Beast Mode) is incomplete in my opinion. They have decided that the reason could be one of: health issues, declawing issues, litter box issues, or unspecified behavior issues.
The reason that comes to my mind first is one of separation anxiety from the boyfriend.
The older cat feels anxious being away from the man of the house because he has formed a close friendship with him. When the boyfriend is out all day working (I’ll presume that) the cat seeks reassurance for his anxiety and merges his scent through his pee with the scent of his human friend, his laundry which smells strongly of him.
In doing this the cat feels he is emotionally with his human companion which is reassuring….continued below the adverts…
From the human standpoint it is a warning sign that the cat is stressed. The cure is to be with the cat more often which may be impossible or impractical.
It all points to that for me because the cat has selected the man’s laundry specifically. If the litter trays were inadequate for the cats he’d likely pee anywhere outside of it.
The health issues and cat rivalry can be eliminated because they are known to be healthy and get along.