"That ring is the only thing you've given to me for real!"
I had a good feeling Saito wasn't going to get mad and becoming more like a true husband he simply offered to get her a new one... but it's hard to argue against the effort and desperation she went through to find that one ring he worked to get her. Fortunately for them he had the skills to figure it out.
I do hope this was an honest mistake and not due to sabotage or another ploy to get them closer
He put the ring on her left hand!
Good practice for putting on the real thing one day.
I only needed to see the new girl's hair tone to know this girl is in all likelihood Akane's little sister, who probably got split up from her via parental divorce and remarriage, then lived abroad. Or maybe just lived abroad to get treatment for her illness. I'd be surprised if she wasn't.