Is it Islamically permissible for me to marry a man who is already married, given that I am a friend of his wife?

If a religiously and morally committed man, capable of providing housing, sustenance, and justice, proposes to you, there is no objection to marrying him after performing Istikhara (seeking guidance from Allah) and Istishara (consulting others). Your acquaintance and friendship with his first wife do not prevent your marriage to him nor do they make you a betrayer. As for your previous relationship with him "within the bounds of a brother and sister," it is impermissible and must be severed because he is a non-mahram (definition) to you.