Melissa,
Progesterone inhibits prolactin, but the prolactin effect on milk synthesis
declines over time so that after 3-6 months is rather unimportant. Prolactin
appears more important in building the alveolar cells during pregnancy and
early postpartum than maintaining their functioning long-term. Jan DeCoopman
wrote about BF after a pituitary resection in the JHL which confirms this
theory.

It may not be the BF that is preventing her from getting pregnant, and may be
totally unrelated.  Therefore weaning wouldn't help.  Have her other hormone
levels been measured?

I don't know about the effect on any prolactin that might get in the milk
itself - most 2-year olds are getting Lord-knows-what in their other food
anyway. She could ask the Ped about any deleterious effects from oral
ingestion of progesterone that might be observable in the baby. I've never
read of direct negative effects on the baby - the usual problem is quantity,
not quality.
Linda Smith