The theatre I work at encourages its staff and board to work elsewhere. Not all at the same time, obvs, because then there'd be nobody to batten down the hatches at home. We find that we learn the best new tricks and techniques when we work with a large and varied groups of theatre peeps -- we can bring those lessons home and apply them to our own productions. Also, when a theatre company becomes too insular -- when its the same folks over and over on stage and back -- it discourages outsiders from approaching (robbing us of their potential talent and awesomeness).

What's wrong with having a direct and honest conversation with this person? If they are your friend and your colleague, then you should throw everything out there and find out their side of the story. It seems like you're guessing about their motivations -- just ask what it was they were looking for by doing shows at another company.