Why would taking three years off be any better during residency than before. It seems more important to not interrupt the on the job training portion of her training? The things she forgets during this time will come back to her over time. Would you rather she have to re-learn the approach for a c-section and the management of polycystic ovaarian syndrome?
To the OP your friend should check the FREIDA site to see which residencies offer a part time option (I have seen it as a search option, but I don't know what specialties have it). I am not sure where you would find info on how long board scores are good for. If she has competitive scores, I think she can get in but some programs may question her dedication to the field which may hurt her. She would probably want the programs that are ok with kids anyway.