Help look after everything else for her so that she has as much head-space as she can to devote to this particular issue.
Stick the 1800 mum-2-mum number near her phone and make her promise to call it before reaching for the formula.
Point out that it gets easier - mouths get bigger, babies get less floppy, Mums get less exhausted.
And lots and lots of hugs for her and bub!