Wrong. There are lines which, once cross, end relationships, and people who know where their lines are, and end relationships when the line is crossed, are acting appropriately and have nothing whatever to learn from you.
He tried to accept her as an SP but when he saw her menu he realized he simply couldn't do it, and as a mature adult did the right thing and cut it off before too many people got hurt. Congratulations for him for freeing both of them to move on to other people--now she can go look for someone who does accept her.
Sounds to me like he's sad the line got crossed, but he knows where the line is. His grip on relationships seems, to me, sounder than yours.
Maybe you are the one who should learn from him.
I'm not saying there is anything wrong with dating an SP, just that it's not for some people. It's a sign of maturity when you know that it's not for you, and you put a stop to it. Not to do that would result in a bad, stressful, and unhappy situation for both of them.
What you are urging him to do sounds like co-dependence.