She means that Dancy took refuge in a pistol which he thought "keeps faith", but his friends including Margaret, also could keep faith i.e., prove loyal to him. But their loyalties were not enough for Dancy.
Considering the reckless nature of Dancy, it can be concluded that his friends could have done nothing to save his life. They all tried their best to save him but he had more faith in a pistol than in his friends.