If a girl gets engaged... (etiquette question)?

If a girl gets engaged who once previously I was very good friends with and talked often about married life, weddings, etc. but completely ignores me now, should I congratulate her?

Some details on the friendship:

Our boyfriends are (were?) best friends and that's how we knew each other, so I also know (knew?) her fiancee very well. He doesn't talk to me either but still talks to my boyfriend often.

We talked all the time about dream houses, dream weddings, where in the world we want to raise a family, how our boyfriends would be each others' best men for sure, etc.

She has the tendency to invent disputes.

She has the tendency to be deceptive....

...So she could have just been pretending to be my friend the whole time(though I feel that that's a huge stunt to pull for no reason. We were friends for two years).

She suddenly, within a week's time, completely cut me off from her life with no warning or confirmation, with no obvious reason. She stopped talking to me, de-friended me on Facebook, stopped following me on twitter, the whole nine yards. I mentioned it once and she pretended like nothing was changed or wrong, added me as a friend on Facebook for a month and then went right back again. This was more than 6 months ago and I haven't heard from her since.

At this point I don't care, but I don't want to seem rude. I definitely don't want to claim her coldness as being my fault (and I feel like not saying anything might be the same thing). And I definitely don't want things to be awkward if I end up at the wedding.

Should I congratulate her or should I give up? At this point I'm leaning towards giving up just for how ridiculous she's been...