The Celtic Triquetra means a lot of different things, namely "body, mind and soul." The Trinity Knot is a Christian symbol that was stolen from the Celts and made to mean "The Father, the Son, and the Holy Spirit."
The girl who ordered this was pretty aware the symbol was of Celtic origin. Again - I just drew the tattoo, she said it was called triquetra. If she knew about the story, why would you just assume she wanted the trinity knot and not triquetra? And then again - maybe she didn't really know either and just liked the symbol (doubtful, but it's possible). Anyway, I'm just an artist. Art alone is more important to me than what anyone decides to call it.