Hello Allybean
What a shame that this has happened. Fully understand your partner's instinctive reaction, but it can be very damaging with cats when they become afraid of their care-givers, and you have to work very hard to try and regain the trust.
My first suggestion would be to put out two litter trays - and if you have the room - three litter trays. Fill one (or two) with the cat litter the scared cat prefers, and leave the other as it is for your other cat. I recognise that it's not often pleasant to have loads of cat litter trays around the house, but generally it's better than having your belongings peed on.
My second suggestion would be that your OH spends time with your worried cat, playing with her and providing her food.
This really needs to happen two or three times a day at least, to reinforce the positive association with her. It would be best if he was the only one to feed her in this period, although appreciate that may not be feasible depending on working arrangements etc.
It's worth considering where this kind of situation arises, that cats in many respects are no different to children. They will do things which may seem to us, at first blush, unacceptable and unpleasant. They normally do it, because they're trying to draw to your attention that they need something from you (in this case a tray with a different litter base).
We can be a bit dense at times in picking up on what it is they're trying to tell us, or we may feel it's not in their/our best interests. But just as we wouldn't exclude, shout at or instil fear into a child in a healthy parenting situation, we shouldn't react to cats in that way.
I suspect your OH already feels bad, and genuinely wants to retrieve the relationship with your scared cat, so he needs to repeatedly show her that he is sorry. Best way to do that is the positive association method.
If she runs away from him initially, then he can try just sitting with her - maybe she wont allow him to get too close at first, and may get up, move or run away. If that happens, he can try just sitting in the same room as her, and shifting position so he gets a little closer each time. He can read, or listen to music but otherwise ostensibly be ignoring her, and have a few of her favourite treats or food tit-bits to hand, so that he can gradually coax her to him, if she makes a move of her own accord. This may take some time, so he has to be prepared to stick at it for it to work. It's best not to force the interaction, and to allow her to make the moves.
Depending on the character of your cat, she may never trust him again to the previous degree, and you may have to be prepared to accept that, though it's to be hoped the relationship can be repaired.
I would also try using plug ins, such as Pet Remedy or Feliway - if you live in a house, I'd put one upstairs in the place she spends most time, and one downstairs. You can also try Zylkene, sprinkled on her food, to calm her nerves. You can get Zylkene without prescription, and it's safe to use. You can also get a pet remedy spray or a feliway spray, and you can spray that on your hands and your clothing, the cats bedding etc. It smells a little like sweaty socks, unfortunately.
Really hope that this can be turned around, and sending lots of "stick with it" and "good luck" vibes your way.
