Very sorry you have to go through this.
I agree with the others on this. Ask about payment plans, lower costs, or Care Credit - all worth looking into.
To answer your question directly, the best, safest, and most humane way to euthanize a reptile is having a vet put it down. Reptiles, and snakes in particular, have very slow metabolisms. This can make euthanasia tricky. It is best left to a vet with the experience and resources to end the snake's suffering quickly and effectively.
I hope you get some answers soon and can help your friend. I am truly sorry.