What is the halacha behind gifting tefilin? What is the halacha of receiving tefilin? Can a Jew gift his only pair of tefilin if he does not use it and can another Jew accept a gift which leaves the receiver without any tefilin?
Like I said in a comment on the question, I don't know if halacha discusses this question, but it just feels like a bad idea to burn bridges like that. Even if the tefillin aren't in use right now, but if they're given away, then the opportunity to start using them again is gone.
Some take a very Toy Story view* of mitzvah objects, that it's "sad" or "unfortunate" that a pair of tefillin or a tallis hasn't been in use (which might or might not have some support in certain minhagim), I think it's even sadder when a Jew doesn't do an easy mitzvah, and doesn't care to the point of giving away the mitzva object to be used by another.
* that "toys deeply want children to play with them, and that this desire drives their hopes, fears, and actions." ~Wikipedia