That's an interesting one. That happened to me when I was in college. Only it was not an ex. I was actually dating the girl at the time. She was on birth control, but had fucked up the timing. We were 2 years into the relationship and she was about to graduate (a semester before me). She already had a job lined up and we were going to be long distancing the relationship the last semester.
She initially got the abortion without telling me. But I could tell something was wrong. Took about a week for her conscience to get the better of her. I was not angry that she got the abortion. That's what I would have wanted her to choose. But I was sad, very sad that she had to go through it on her own. We ended up breaking up about 6 months later, but that episode did not have anything to do with it.
Let me bounce that back to you. Which would be harder to forgive in your eyes, a woman who did it without talking to you about it, or doing it anyway after talking to you and hearing your protests?
That would be a hard conversation though if you would have opposed it. Did you go all Michael Corleone?