Few people other than bat specialists know much about bats, so that was a bit of a cheeky comment from the rescuer. It sounds like the fact she was a baby meant that she would die anyway away from her mother, and we don't know how long it had been that she was abandoned. You gave her a chance
and if you have a sick bat again you'll contact the rescue straight off - and so will all of us who've been following the tale.
Very sad for the baby, but overall something positive has come of it, and of you sharing the problem