Just want to add my $.02 worth from another perspective.  If this mother
is on glucophage, she has Type II diabetes (formerly known as adult-onset
diabetes, usually not requiring insulin as the juvenile-onset - now known
as Type I - which always requires insulin).  Type II diabetes is extremely
amenable to diet, weight loss and exercise.  Often with such
measures, medications aren't even needed.  Also, people with Type
II diabetes have a lot more leeway with higher blood sugars - i.e.
they are not in danger of diabetic ketoacidosis and hospitalization
if their sugars are high.  They need to keep their sugars in good
range to prevent long-term complications, but are unlikely to be
hospitalized for serious complications if their sugars are not optimal
(there is a complication known as hyperosmolar coma, which can occur with
sugars above 400, but with careful monitoring, this should be
preventable).

Unless her glucose levels have been off the wall, there's a good chance
that breastfeeding alone could lower her blood glucose levels to
acceptable - even normal - levels.  Breastfeeding consumes 200-500
calories/day, so if she eats a reasonable diet and has a gradual
weight loss, she may be able to stay off the metformin while
breastfeeding.  There is a study which showed that women with gestational
diabetes had optimal glucose control when they breastfed.  So why use
metformin?  Why not try her without medication and see what her levels
are?  If they are in the normal range, great!  If they are a
little high (upper 100's, low to mid-200's), she can work a little
harder at weight loss - perhaps add extra exercise.  If they are
consistently high (above 300) and don't respond promptly to such
measures as diet, exercise and weight loss, then medication may be
needed.  But remember, unless she is symptomatic from the elevated
sugars or they are at a high level, there is no urgent need to
keep tight control during the relatively brief duration of
breastfeeding (as compared with several years of abnormal sugars
needed to cause major complications).  Regards, Alicia Dermer, MD, IBCLC.