Advice on a weak, tiny baby kitten

A friend of mine has had a couple of abandoned kittens dumped on her
doorstep, and she is attempting to take care of the poor little things.
Both were skin and bones and had swollen bellies and were infested with
fleas.

She has bathed both of them to help with the fleas and is feeding them
kitten formula. One of the kittens seems to be doing fine on
the formula but the other is not at all eager to take it's formula, cries
a lot, and is listless and non-energetic.

Other than a trip to the vet for a checkup, any advice on what might help
the weaker kitten?