The main focus of a defense that could be mounted for these individuals would have to be on predicated on the lack of motive--that is, there was no intent on the part of the individuals to inflict serious injuries on the victim, and certainly not death.
The problem with such a defense, however, is that the theft involved an assault. Pushing an elderly woman to the ground so that a crime could be successfully perpetrated will likely be considered aggravated assault, given the woman's age and the defendants disregard for an injury even simple assault could cause, which probably rules out a charge of involuntary manslaughter.
Depending on the prosecutor, a charge of third degree murder is not out of the question, as the jury will be very sympathetic. I could probably avoid conviction based on the lack of intent, but all the prosecutor has to do is establish that the defendants committed the robbery and assault, and they would likely get a conviction on any charge lesser than murder.
Some might be tempted to highlight that the woman resisted the theft, but try selling that one to a jury and you'll be in trouble. I think the best chance for an acquittal would be to raise reasonable doubt that the defendants were at the scene at all.