+: sorry, but after much discussion with my significant other, I can't take the decision you want me to
me: well, it's your decision, it's your life... just to state the fact, I think your significant other doesn't know what he's talking about
+: yes I argued all night but I promised to respect our joint decision
me: ok. I think you made the wrong decision but I will bless whatever you decide :) and I'll prove your significant other wrong!
Part of growing up is letting go, and letting your friends make their own decisions. We have to make sure they are informed decisions, but at the end of the day, it's their life. Anyway, I could be wrong about what I think, right? We may love our friends dearly, but we have to acknowledge that we are not responsible for them - they are.
So good luck... because you'll need it ;)