Obviously this is a more extreme example, but following that train of thought to its logical conclusion, you could just as well justify cheating as long as you don't get caught - after all, what your partner doesn't know can't hurt them!!
I think the person who found the courage to write that letter would be devastated if she found out it was used as a plaything for somebody else having sex with her crush, so it's a jerk move in my eyes, even if she doesn't find out. And like... it's not like it was necessary to do that, you know?