Your co-worker, a junior, quit your C# shop to go work someone else doing something else for more money than you? Who cares about your title at that point, just go get a junior job in a different language and restart. It isn't like you're losing money to make that change. I don't see the problem here. It's a different story if a change would cost you cash, but if you can actually make more money from the switch, I don't see the issue here, unless you are super title-concerned.

J.Ja