I am sorry for your loss.
I know it's difficult, but the situation needs a quick reaction, do not let the joey with the corpse, aside from the fact that the cause of death are unknown and possibly contagious, the joey will get hypothermia really fast without her mother's heat.
Depending on her age, you will have to feed her manually with a syringue (just put a drop of milk on her lips and let her lap, do not force the syringue in her mouth). The best replacement milk is wombarroo milk, but you can use puppy milk too (not kitten milk).
I advise you to read Suz's page about handraising joeys, it's full of informations and explains the process better than I could do:
http://www.suzsugargliders.com/handraisingajoey.htm
On a side note, I am not sure your vet is knowledgable about gliders... Milk fever has never been reported in gliders, and in any cases, even if it existed, it would not apply in this case because basically milk fever is sudden hypocalcemia caused by the 1st production of milk, which implies that it would occure the first time the joey feeds (right after birth when she went in the pouch) long before you would even notice her presence, it would certainly not occure with a fully furred joey who is already out of pouch.