There is a field you can request that Facebook include in the friends request. I had an app that did this:
local params = { fields = "name,username,first\_name,last\_name,picture,installed" } libFacebook.request("me/friends", "GET", params, onRequestComplete)
See the “,installed” in the fields list?
You can then look in the table of returned people and filter out people who don’t have the installed value.
Rob