I agree that if the mother really wants to do it, she can surely try.
As part of the process, however, I would gently explore with this mom
her feelings around breastfeeding having ended at 7 months and her
reasons why she wants to relactate.
One of the things that comes to my mind is that a 19 month old may be
starting to come into "his own mind" about many things and may not be
altogether willing to go to the breast.  How many times have we seen
happily nursing toddlers come off the breast (just as the milk is
letting down :-/) and bounce off to investigate other goings on?  This
mom's child has spent more of his life NOT breastfeeding.  I don't mean
to be discouraging, I just think the mom should understand that these
factors could make relactation (already a challenging endeavor)
particularly difficult.

Diane, LLLL in VT