I say go to the farewell party.
Since this person is your wife's best friend, that is enough reason to go. Addtionally, that likely means that this person was a friend to you also, and you will not see her often from here on out - if you DA or are DF'd you may never see her again.
Plan to attend the Meet-up the following month. Mark it on the calender and tell the wife that unless something crucial comes up, you have begun to establish a bit of a friend base outside the bOrg, and are going to a social gathering. [All true. It might even raise some interesting discussion.] If you enjoy the meet-up, tell her that you did, and that you might just attend a monthly get-together of this group. You could keep the XJW part out of it - though it might someday come up. You will surely meet some people there that share some interests with you, and can begin to grow away more and more from the Jw scene.
Both have merit. You obviously love your wife. Show it on this occasion. Perhaps letting her know later that you attended, not begrudgingly, but at the sacrifice of another social event. I would not tell her beforehand - you don't want to send the signal that she drug you to an event in which you had no interest.
Just my 2 cents.
Jeff