The Devils have been looking for a coach since they fired Hynes in December. They probably asked the Rangers for permission to talk to Ruff some time after the freeze. First, I don't think the Rangers would have turned down the Devils' request even under normal circumstances, given Ruff is a respected, veteran coach who obviously wanted to get back to being top dog. Second, given that no one really knew if the season would resume, it would have been really unfortunate if the Rangers declined to let Ruff talk to the Devils until the season was over, only for the Devils to hire someone else and then for the league to cancel the remainder of the season.
Whether Ruff finishes it out, I don't know. I can see why the Rangers would want to keep him on with the playoffs going on and all, but if they have someone they think could step in (or if they think it would be manageable without another AC stepping in), they may feel it's better to just let him out of his contract. It's tough to expect a guy who has accepted a new job to be fully focused on the matter at hand when he has one foot out the door.