Anne,

Your client will produce colostrum following the birth of her new
baby.  Her mature milk should come in at two to three days postpartum,
just as it would if she were not nursing her 22 month old son. You
are correct that her milk will be ideally suited for the newborn
(premature or not), though it will also be fine for her older child,
unless of course he gets diarrhea from the sudden bounty of milk.  An
additional benefit: the older child can help out with any engorgement
mom might experience.

Hope this helps.
Dorothy Rae Collier