I would give them the ability to invoke a specific patron (probably making a designer table) but not grant patron bond in this case. That way, they would not gain the ability to bond others to that patron, or to bond to other patrons. I would also rule that access to the spell is not the same as learning the spell, so it would not use up a spell slot, and would require the caster to keep the horn on his person.
That's probably a better way to do it. Maybe it's a very limited form of invocation, perhaps only really useful for summoning similar demons (which, if the user isn't prepared, could end up being highly entertaining...).