Oh my, I'm very sorry for your girlfriend's budgie current condition.
Under these circumstances, and given the fact Butch is getting weaker, you really need to take him to the avian vet ASAP, this is an emergency situation and Butch may need to be hospitalized.
Has the wound stopped bleeding? If not, you can stop the bleeding by applying corn starch on the affected area.
Since Butch has lost a lot of blood, it's important to have some of his favourite food in hand, you can offer him millet spray and soak in it electrolyte solution. This will give a much needed boost to your budgie.
Below is a link with all the detailed info.
Once Butch's injury is no longer bleeding, you can carefully wash some of the blood off his feathers with water or sterile saline solution and you can disinfect the injury with Betadine.
Betadine also speeds up the healing on Butch's wound.
The fact that his poop is watery is perfectly normal, given the fact he has been under a lot of stress from the whole situation. You can cover his cage on 3 sides with a fleece blanket and leave the front part open. This will make Butch feel more comfortable and will keep him warmer.
I hope you will be able to take him to the vet very soon and I'm sending my best wishes for Butch's steady and full recovery.
And despite the circumstances, welcome to the forums.