Wow, first let me send my sincerest prayer out to you, bless your hearts -- all 3 of you. I am shocked that your friends think it's funny for their son to be such a bully, sounds like a big cop out on their part... I would not invite them around again unless you have a heart-to-heart about his behavior -- unacceptable!!! I have an 18 month old daughter who is very gentle and very friendly and it hurts my heart when other children don't respond to her in a healthy positive way but unfortunately we can't do anything about someone else's child's behavior. And it is a tough life lesson so early for our dear little ones...
I think all you can do is to continue to reassure her that the world is not made up entirely of bullies and I would let her come out of her own "protection zone" when she is ready. I find that my daughter tends to "teach" me about her readiness for each step we take and it is my responsibility as her mother and caretaker to let her emerge as she feels like it/ready for it.
Perhaps one of these days when your daughter is ready, the next play date will hug her and she will know not everyone is so horrible. I picked my daughter up from her "school" one day and they told me that they had had a new child in class that day and that my daughter followed her around patting her on the back and "talking" to her all day. I loved hearing that about my sweet baby. You should know, there are others that will be kind to her. And I would just give her the time to heal and find that out...
Best regards to you and your daughter. And I think perhaps next time, tell your friend to get control of her "boy" or you all are leaving.