This drug is not listed in Hale's 1997 edition.  However the CPS
(Canadian PDR) says the following:

Stable state blood levels in the person taking the drug are achieved
after about 5-6 weeks of taking it.  They are usually in the range of 39
to 115 nanograms/ml, while taking 10 mg daily. (yes, nanograms, or
billionths of a gram).

The drug is 90% protein bound, and another 9.1% is bound to red blood
cells.  This is good because only unbound drug gets into the milk.

Part of the reason for the low blood levels is the drugs large volume of
distribution (most of it in the body is not in the blood).  This means
that it will continue to appear in the milk even for some time after the
mother stops taking it, but again in very small doses.

Given this info, I think, if there is nothing else the mother can use,
she should continue breastfeeding and take this drug.

Jack Newman, MD, FRCPC