I'm very sorry for your loss, you have my deepest sympathies over Bluey's passing.
From your description, it really seems that Betty is vomiting.
Besides problems with related to digestion/crop issues, I doubt he would have such adverse reaction strictly from mourning and I would say that something else has triggered the bout of vomiting. Betty hardly had the time to realize he no longer has Bluey since this all happened today.
The non-stop eating and stuffing himself so fast can certainly cause a digestive upset.
Did you only happen to notice this change in Betty today? Has he been eating and drinking well before today? How were his activity/energy levels?
Have his poops been normal for the past few days until now?
For now what you can do is to keep Betty extra warm by covering the cage on 3 sides. If need be, you can also place a heating pad or a hot water bottle underneath cage's bottom.
In order to hopefully give Betty a little boost, you can add electrolytes into his drinking water and try to offer it to him. If you have a pipette, you can give it directly to the beak. https://talkbudgies.com/your-budgies-...te-recipe.html
Vomiting spells can take a lot out of a budgie depending on the severity of the bout. If Betty isn't coping well, is overall lacking in energy and if he vomits again, then it would be best to have him seen by an avian specialist right away.
I'm wishing Betty all the best.