Thank you, Dr Saab! 10 years and where are your other students! LOL
I do not follow this advise, but I once read that you should never tell someone they are wrong, this usually ends up with them totally ignoring what truths you are trying to convey to them. Better to throw in hints here and there and let them work it out for themselves.
With this in mind, know the truth and drop these off at the appropriate, timely, opportunity, for example, Quran says this, what do you think this means Uncle? etc
There are many good people who follow/believe nonsense. On this earth they may move two steps forward but the Quran says inevitably the wrong system will cause its own destruction. Judgement on this earth will be passed. What good they do will be judged by Allah in the next life so let us leave this to Him as He is the Best of Judges, Most Forgiving.
Marrying muslim mushriks is a necessary evil, better than marrying non muslim mushriks, but finding someone compatible may lead you to straightening the crooked rib.