I thought about removing the PK, or using a solid container.
Honestly I don't know whether bettas are stressed out by seeing another betta, none of the ones I've kept seem to have problems with their reflection :O
my biggest problem is moving him to another container, In past experiences I think that stresses them out more than anything :(
So I try to keep them in there. I had an idea about putting a black plastic bag around it. but I'm not sure how safe plastic bags are , lol. I'll see what I can do later of he's still alive
I had a very similar case a while back, I think the ventrals are shrivelled. not sure what causes it