Shag-adjacent for sure.
If as a PP says, folk from SA are more comfortable expressing their ‘territorial’ or possessiveness, then wearing your DH’s shirt is her scenting him by proxy. It is just so ‘off’, a of her taking him as she pleases as & when.
Your discomfort is thwarted rage & indignation at this overstepping the boundaries of good manners & her making a move on him.
Where is DH’s friend on this matter? Can he not read the shirt buttons? Another man’s shirt on his partner’s back ?
It’s all a bit too intimate: your DH’s smelly shirt in with her wash, her taking it to wear as she pleases. After all, how would feel is she did the same with your underwear or clothes if she popped it in with her wash?
When we wear someone else’s clothes, we borrow something of them, we mark ourselves with their scents, their warmth, protection & security. We leave our scent & shape therein when we give the boyfriend’s jumper back.
Who hasn’t kept an item of clothing of some one we miss deeply, e.g. baby clothes, Mum’s favourite scarf that wafts her scent, Dad’s old jumper. Maybe it’s me?😉.
I agree with many, it’s shag-adjacent.