["It's always sad for me to see a friend from work, leave and get a new job.", 'Yeah that is pretty tough. Especially when you really like to work together.', "Yeah! It's one of the main reasons for liking a job, is the people you work with and become friends with."]
Then you have to meet their replacement and try to get to know them better. It usually isn't the same.
It's always sad to see someone you liked from work find a different job.