[RobinP]

"Finally, we for the first time demonstrated that progestins favor development of breast tumor metastasis via PR function as activator of signaling pathways. Our present findings provide mechanistic support to the design of a novel therapeutic intervention in PR-positive breast tumors involving blockage of PR capacity to activate cytoplasmic signaling.
PMID: 17440047 [PubMed - as supplied by publisher]"

Many with her2+ bc have hormonal negative cancers which means that the tumor neither has estrogen or progesterone receptors. I would assume that only pr+ sensitive bc with progesterone receptors would be sensitive to progesterone. Also, I would think that eliminating progesterone would only benifit those with the progesterone receptor.