Disney will post warnings where appropriate, but not actually stop her from going on light rides.
As long as she's not doing it as some sort of stunt, hoping the baby will be born in the parks, and her doctor says OK, I understand her desire to go. Her other children are probably looking forward to this special time with Mom before the baby takes all her attention for a while. Watching my kid have a good time, and just soaking up the Disney atmosphere, is as much a part of Disney to me as rides.
She will, of course, have to adjust her expectations and listen to her body (especially staying hydrated!)