Good luck on receiving great advice from other moms who are going through this.
I can only say that my daughter has started this as well but like yours is very very easy going and accepting of everyone so it hasn't impacted her greatly ... yet. I however haven't forced her to hang out with the other girls except on occasions where i can ensure that the activity they are doing (beach, ruby's, whatever) EVERYONE can enjoy. So the mom and kid still feel included in our lives but not in a forced way.
The main thing I have learned as a mom since DAY ONE, think of your child first. Sorry but since I'm an older SAHM I guess that's all I'm about. That is, I never took my kids shopping when they were tired or needed a nap. Therefore, I never once had the incidence of the screaming baby while I was trying on shoes. etc. You get my drift. Same goes in "forcing" friendships for your own sake.
Sorry it came to this. Wish we could have all helped out before the friendship (may have been) ruined. Hope you get some good tips.