Legal Question in Personal Injury in California

Family members generally don't get to make decisions like this for another adult. That's true even if she relies on them for her care.

Is your friend's grandmother still competent to manage her own affairs? If she is, then it's up to her whether to sue even if her family disagrees with her decision. If the family can prove to a court that she's no longer competent then they could have her placed into conservatorship. That's a drastic step (and an expensive one), which would give the conservator control of *all* of her affairs and not just this particular decision. Whether to sue would then be up to the conservator -- again, even if the family disagrees with the decision.