I'm so sorry for the loss of your kitty.
The grieving process can be quite long - mine grieved for over a year, and her personality has permanently changed, and I agree with lovingfurballs that going back to the food she was used to is probably your best bet to get her eating again. The new food coupled with the absence of her furry buddy might have been too much change at once for your kitty. Once she's eating well, you can try reintroducing wet food again.
My kitty was also not eating after her kitty companion passed away (both were about 14.5 years old at the time). She's always had issues with constipation, and stress always makes matters worse. It was hard to tell whether she wasn't eating because she was constipated, or whether she wasn't eating enough to pass much stool. Also like your kitty, she lost a lot of weight. Part of that turned out to be a reaction to her flea meds, but the other part was her anxiety and grief. She was overweight at nearly 9 lbs, and lost nearly 3 lbs in 2 months - so roughly the same percentage as your kitty - 25-33% of her body weight. That's enormous. While the circumstances are the likely reasons for the weight loss, you might consider a vet visit just to be safe, especially as she is an older cat. The vet could also prescribe an appetite stimulant, like periactin. Or you could try treats, like freeze-dried chicken or tuna that you could crumble on top of her kibble (or on top of wet food - maybe that would entice her to eat wet), a bit of boiled or roasted chicken, plain chicken or turkey baby food, sardines...anything that she might find appetizing.
I tried a pheromone diffuser and homeopathic drops (HomeoPet's Anxiety Relief, but there's also Bach's Rescue Remedy) added to her water. I think they did help a little, but it just took time. You might find that she's needier and needs more reassurance from you, so just keep giving her lots of loving as you have been. My kitty was always a velcro kitty, but after my other kitty passed away, she's become even more clingy and very needy. Keeping her active and mentally stimulated with toys will help too.