I would actually not give any. It would be beyond me to influence someone else’s choice of partner. I doubt that I would even try this with my sons. If it had any effect at all, it would be either to backfire or to alienate the other person, or both. Is there a subtext here—i.e., that you think your friend has made the wrong choice, and you want him or her to figure that out before it’s too late? If this is just speculative—real friend, hypothetical engagement—it seems like this would be a fine subject for the two of you to discuss in the abstract.