She used the HOV lane for her convenience, not to reduce the number of cars on the road. The lanes are not there to make travel easier just because one is hauling kids. Same with you having your blind friend as a passenger. Using the HOV lane makes your life easier, but does not take another car off the road.
In both situations, if you were to honor the spirit in which HOV lanes are intended, you should be in regular traffic and leave the lanes to those that have actually lessened the traffic load.