Daniel Doyce returns from Russia to find that Merdle's bank was a fraud, they've lost the money Arthur put in the bank, and Arthur is a debtor in the Marshalsea. Arthur understandably worries that his business partner will blame him. Daniel doesn't. And he made enough money in Russia to make up for the money they lost.
Daniel is just a nice guy. Nice enough to escort Affery at Arthur and Amy's wedding despite having no idea who she is. Watching them walk arm in arm, Affery's face absolutely RADIANT with joy after being creased with fear so long, is overwhelmingly sweet.