Actually, I don't think it's gonna be NTR in either direction actually. I don't know about Japan, but with very few exceptions, wedding rings don't have stones. Most likely that's an engagement ring, and the signs point to it being there because she's hung up about that relationship ending.
If she's actually a vampire, since the rash suddenly popping up is pretty extreme, then he either didn't believe her or didn't want to deal with it. if it hadn't ended at some point, she wouldn't be losing weight because either she'd be eating real food, or would have the blood she needs as a vampire.
If she isn't and it's just some psycological trauma, the above remains the same really.
Either way, I'm pretty excited to see this develop.