I agree with xuhuanyu that you have captured the meaning of the sentence, but you may like to consider the following changes:

They used to be classmates. Now they are colleagues. If only she had agreed to be his wife.

"Previously" seems redundant since we know it's in the past from the past tense. "He was her classmate" is 他以前是她的同學 literally, and it is slightly different from what you have. The correct direct translation is "he and she were classmates", but I think a simple "they" will do the trick. The last sentence is only my personal preference. In your translation the action is indicative; a positive would yield if the action was taken. In my opinion the sentence implies an optative mood, where the speaker wishes her to marry him. I have chosen "if only she would..." to express that, and there is no need to say "it would be good" since it is already implied.

Having said that, I am overly scrutinizing the translation and I think your attempt is perfectly fine. :)