Dan Savage of The Stranger's LoveCast has a lot to say on this so although it's usually directed more to LGBT people/relationships it fits here as well.
To use his general thoughts: Communities are small worlds and poly is definitely a small world. Even from the rather large metro area I live in the poly community seems almost like a soap opera at times (excuse me 'daytime drama', lol), lots of switching of partners and dramas and, like you're situation; I don't want to remember you're alive type people.
As Dan Savage says, since we're all part of that community and its small, it isn't fair to 'ex-communicate' someone from it (without serious cause). His suggestion is rather than completely avoid the person you acknowledge them and the situation. If/when you run into the person you state out loud about the awkwardness. "whoa, this is awkward. Know its tough but we have friends in common so we may as well make the best of the situation. Let's just try to make it easier on both of us...."
Or some such, Dan said it much more eloquently I'm sure but you get the gist. She has no right to ex-communicate you from the same small community you both are a part of. We alternative lifestyle people need to try to remember its can be challenging for us all and help each other out.
And she always has the option of staying home
(I also really appreciate how Dan advised if anyone makes you choose between friends b/c they're upset about what a person did; he'd choose the one that didn't make him choose