I got a surprise bike for someone once, and it was a disaster. I wanted so much for it to be a surprise (the moment), but the bike didn't fit and he never rode it.

I would encourage you to cut out a picture of the bike as the surprise, then go get her properly sized. That does not mean simply standover height. It means looking at the top tube and stem length combo and crank arm length, too.

A 49cm could still very well be too large. I am a hair shy of 5'6", and my bike is a 49cm. It just depends on how a person is built, not just height.

It would be a shame to get her a bike, it doesn't fit, she is uncomfortable, and so she never gets into riding and then stops and shies away from ever riding in the future. All because the bike doesn't fit.

It's very nice of you to get your wife a bike, by the way. I hope that whatever bike y'all get, she'll have lots of wonderful miles on it.