I don't think you have to go to extreme measures. Simply having a talk with your friend about how his behaviour is inappropriate and very damaging to his family will suffice in the beginning. If this doesn't work, then you can escalate the resistance.
The key I think is for you to understand what is going on here, to get a transparency on what's going on. It's not true love, it's emotional infidelity. It's not platonic relationship, it's emotional cheating. He is not a prince charming, he is a bitter individual with clear issues. No happy, sane married person professes his love to other women and becomes clingy and emotionally attached to them.