For her specific situation, it would be best to talk to her personally (even her doctor and her) about what type of long term consequences this will have for marriage. From what I understand, some women who have mrkh can have biological children- just not without assisted reproduction methods.
There are varying degrees of this condition, so it would be good to know exactly what degree this effects her now, and what the surgery should help with. It's ok to gather information and try to process it- just as I'm sure she needs to process what this means for her.
I know getting into a marriage with someone with a medical condition can be nerve wracking- but keep in mind that you could marry a completely healthy girl and then something could happen (accident, illness, etc) that may give her a condition that changes that, even after marriage. Ultimately, you need to decide if you are willing to stick by this person no matter what the future holds- if the answer is that you don't think you can, then don't string her along- if you think this is the girl for you and you know that you will stick with her no matter what, then keep the relationship and work on communication and game plans on how to deal with this or anything else that might come your way in the future.