While I think that putting her on a restricted list or unfriending her is perfectly ok...especially considering that you don't interact much online or at all in person, I do think that if the sole purpose of doing it is an assumption that her inadvertantly learning that her ex is getting married would hurt her feelings, then it's not necessary.
The relationship ended two years ago and they parted amicably. I'm also assuming she's displayed no "why don't you love meee" melodrama or stalkerish behavior since then, so as nice as I think it is to want to spare her some possible minor hurt feelings, I don't think it's really necessary.
I've always lived by the philosophy that it's not anyone's responsibility to help someone avoid unpleasant feelings/knowledge. Your father doesn't care that you are FB friends with her, so does it really matter all that much if she sees some posts about his upcoming wedding? If they bother her, she can hide the feeds.
Now, everyone uses FB differently and if you really don't want to be FB friends with her anymore, then it's certainly your perrogative. You can unfriend her for any reason you want. I just think that you shouldn't worry about her finding out your dad is getting married. Mature adults understand that relationships end for various reasons. And to assume that she wouldn't be ok with the knowledge, or would do something hurtful because of it, I think is unkind. It takes away her ability to react maturely (even if there is some twinge of hurt) by assuming she won't or making sure she avoids the emotion completly. It's so nice for you to think about her feelings, don't get me wrong, but I also think that by attempting to do something nice, you could actually hurt her feelings more. At least, if I found out that someone chose to set up a situation so that I never found out about it, because they felt it was their responsibility to spare me my feelings, I'd be more hurt. It would communicate to me that they didn't trust me to feel something and react appropriately to the knowledge. (This is obviously not the same as throwing it in her face...if she sees a post, she sees a post. She'll likely find out about this sometime/some way anyway).