I’m so sorry for your loss. It’s so hard, especially for the rabbit left behind.
I understand what you’re going through as I also now have a lone rabbit who for 7.5 years had never been alone, ever. She was a part of a group of 4 and I lost 3 of them within the space of 3 years. I was down to a pair last year until my frenchie passed suddenly after a battle with his health, he was 6. It’s important I think that they see the body of their friend because I do personally believe it helps them process and understand that their family member is gone so that’s good you were able to do this. I find that sometimes they will grieve for the passes rabbit, but not always. It really depends on the rabbit. My two boys were joined at the hip, and when the first boy passed very suddenly out of no where it hit his brother hard even though he was able to see him and understand he was gone and even though he still had his sister left. While he never went off his food, he definitely became a lot more quiet and withdrawn for a while. And then when he died, his sister was left all alone. She was previously a very rabbit-rabbit. She liked people, but LOVED other rabbits; she’d choose her brothers over people any day. But when she was left alone, she became very clingy and always wanted to be with people – because she was lonely. So some rabbits will become more withdrawn for a while, and others will become more clingy with people – some will be totally unphased. It’s hard to predict, as it varies from rabbit to rabbit. You just have to keep a close eye and act accordingly. Because your rabbit got to see the other had passed, I think (in my experience anyway) this makes it a lot easier for them to process and they take the death a lot better than a rabbit who didn’t get that chance.
My surviving girl has not been cleared by a vet to pair up again and she lost her brother last July ‘23 (complex health problems, I’m really hoping we are cleared soon!). Ideally, you’d let them grieve a little, understand their friend is gone, and then start the pairing process again. If that’s not possible, for my bun I got her loads of rabbit life-like stuffies. It is depressing to watch her groom her stuffies and see her confused over why they won’t groom her back, but she loves cuddling them and grooming them and they offer her some comfort. So I’d definitely recommend looking into something like that from the likes of Amazon or something. Turn a radio on low during the day to also offer some sense of someone being around even when you’re physically not; that’s what I do for my Bailey anyway, and I do think it helps her especially when I’m in work. And then of course, spend as much time as possible with them.