Why did he add her on his facebook as a friend?

[It is only me]

I dumped my ex 2 months ago. Since then we did not speak at all but he kept me on his facebook despite the way I broke up with him which was not very nice.During our relationship he was very possessive and controling and was trying to tell me what Im going to do with my house, money etc.
Our jobs are connected as he deliverers things to where I work.
Now i have noticed he added to his facebook as a friend a woman who is my friend and who works nights where I work. She told me that he never chatted to her before but now he was very chatty and friendly.
I was wondering why did he do that. Any suggestions?