I don't want to be a jerk about it, but I don't want to be a pushover either.
Why do you feel you need to discuss it in the first place? I know we as Catholics are tasked to evangelize, but I personally do not because it's a great way to lose friends and family. The people I know who are masons do not rub it in anyones face- one is a Rabbi. Unless someone is involved with a dangerous, predatory cult like $cientology, I respect the path they have chosen to walk. Having said all that, some people are open to civil debate about such matters and can remain friends.
I certainly would suggest it in as nice a way as i could muster (I tend to be more like an anvil than a hammer, which is also counterproductive). Pray for him and fast and offer him up to our Blessed Mother for his enlightenment. Masonry is insidious, covert and dark. But the good news is that Mama Mary will always crush the head of satan.