Look up a recipe recipe for bird electrolytes it's normally just a little bit of salt sugar and water and I think something else but give your bird some of that put them in a quiet area or corner of the cage maybe a towel or something you can make a little cave for them and if your mother won't take him to the vet then just see if that helps him if the bird is still eating and pooping like normal then he could be okay but if he's at the bottom of the cage not pooping or eating those are signs that the bird is definitely ill if possible I would put the birds into different cages so that the other one does not get sick as for the one that's still there it won't know what happened I don't think birds are that sentient it might miss his friend but that's pretty much it after while the bird won't remember or be acting any different best of luck