@rrk93 I’m sorry for your loss.
It sounds like grief. One of our Boxers would refuse food and activities for a week each time we went on vacations. The diarrhea could be due to stress or due to the variety of food you tried to feed her.
Was she able to see your boxer after he passed? How long isn’t she eating anymore and how much per day is she eating?
For getting her to eat or at least to prevent weight loss
a) try liquid food (eg RC) or bone broth
b) prepare cooked chicken or any other low-fat meat with some carbs or veggies (if she isn’t used to veggies and has diarrhea, I’d give her only meat with carbs)
diarrhea: Add more carbs like overcooked rice (alternatively potatoes, oats, sweet potato) with a spoon of pumpkin, Morosch carrot soup or Olewo carrots.
grieving: Give her extra love and try to get out of the house with her if possible. Some nice place which doesn’t remind her of her friend. I’d also make sure there aren’t too many things of him in the house anymore.
If it doesn’t get better, bring her to the vet.
Hope she’ll be fine soon.