What would you accept as proof or do you just reject the idea of "former Christian" out of hand on theological grounds?
I used to believe in God, trust that Christ saved me from my sins, believed I had a personal relation with him,considered myself Christian, went to church, prayed, studied the Bible and all that jazz for example. My Church also accepted me as a Christian and treated me as such by giving me accesses to the sacraments and such. For that reason I say I used to be a Christian. I don't see why saying that would be problematic or even a particularly suspicious statement . Lots of people are practicing Christian and then move on to something else.
Christians seem to be the people who say things like that too. I never see people saying there is no such thing as an ex-Buddhist or ex-shintoist or whatnot.