You said "he tried the "nice guy" persona in order to get me. when it wore off, I broke up with him "
Well he is being the "nice guy" with her now,thats how they reel you and in the first place.
Some time in the future she is going to be in your shoes,no doubt about it-Guys like this don't change,trust me I was with one for two years.
Ill tell you something else,when I got with this guy,he had just split with another girl,she saw me and she said to me in tears "hes treating you better in a week then he did me in our whole relationship".
Lol,that didn't last.
He also used to say he was emotionless and insensitive-they must of gone to the same school lol.
You also say he's "the best guy ever" suddenly-hes already disrespecting her by talking to you when she isn't comfortable about it,so don't even take it there.
They don't change,because guys like this have serious personal issues,they just go on to do the same thing to someone else,trust me on that,they play the wolf in sheeps clothing,and when she is relaxing in the r.ship he will most likely switch it up.
My advice to you is to stop worrying about what he is or isn't doing and why,and focous instead on yourself and getting over being in a relationship like this-you should be thankful you are rid of him now rather then later,he will never be nything but bad news so walk away with your head held high and forget him-hes someone elses problem nw.