I have said this once before and I'll say it again, unless you have something to gain from "coming out", don't tell anyone, not anyone at all. The very nature of ABDL is a private thing in and of itself and concerns no one else but already like minded people or those it
needs to apply to. Unless you are in a situation like, lets say, a marriage, or that it should be brought up with a therapist because it's affecting your quality of life, then there would be reason to come out.
So answer this: how will telling your friend that you are an ABDL work to your benefit and enrich your quality of life? Is it not enough that you are already part of a community that share common interests, hold meets / conventions, etc? Can you not live this out in a private manner? What sort of reactions do you expect to get from telling those closest to you? Do they need to know? Those are the sort of question you need to be asking yourself. If you're ultimate goal is so you are able to prance around in nothing but a diaper and tutu in front of your friend, then it might cause nothing but friction because ABDL is not his thing. But if you
must say something,
read this. And good luck!