She also is stalling about moving closer to one of her sons, and won't let go of possessions even though she is in an apartment and stuff is stacked everywhere. One of her sons helped her move, and left a bunch of boxes behind to get rid of, but she is gradually bringing them all to her apt.
Should they call some senior service agency to warn the person who she gave money to? They are afraid this person and others will drain the bank account.
If the 'gift' was genuine, she was competent to make it and there was no fraud or coersion, there is unlikely to be any way for her to get it back. The issue is to stop it happening again. Please search the site for all the other options, or suggest that to your daughter. Yes it does matter to your daughter and her own family, because the lady may find herself in a very difficult financial situation and look to them for essential help.
As the family didn't step in to finish off the previous move and actually get rid of the boxes that were cleared up, it sounds possible that things have changed and they should now be looking at taking more involvement in what is going on. It can be a difficult step to take, and it helps a lot if all the family can make a joint decision and have a joint plan.