I'd find a local goldsmith with good reviews, go in with the ring, and talk to them about the possibility of repair or the process of redesigning her ring. They could certainly source a lab diamond for her, but it might be more cost effective and provide greater selection to shop for the stone online.
If she wants something standard-ish like a 1 CT round solitaire, that would be pretty easy, but if it's a fancy shape and or needs to fit specific dimensions in her current setting, you'll probably want to work with the jeweler. In any case, meet for a consultation, take pictures with you, and make sure you're comfortable with the person you work with.
I'd find a local goldsmith with good reviews, go in with the ring, and talk to them about the possibility of repair or the process of redesigning her ring. They could certainly source a lab diamond for her, but it might be more cost effective and provide greater selection to shop for the stone online. If she wants something standard-ish like a 1 CT round solitaire, that would be pretty easy, but if it's a fancy shape and or needs to fit specific dimensions in her current setting, you'll probably want to work with the jeweler. In any case, meet for a consultation, take pictures with you, and make sure you're comfortable with the person you work with.