Background
A colleague (we'll call him Tim) and myself work to support/develop several products, including a mobile application. This guy has been here for 2 years and 8 months (I joined 1 month after him). During these years he's made a very poor impression on me:
He is not capable of advising customers. He almost volunteers any opinions or advice, and when pressed to answer the information he provides is typically not in the context of the problems, or is not a solution to the problem.
He is loathe to pick up any new technical skills, which as you guys may know is critical in the fast paced field of mobile application development.
He is very sluggish, he even types using one finger only (who does this in this day and age?) and doesn't want to learn/use shortcuts.
His code is basically terrible, with a lot of bugs and inconsistencies, to the point where I can't trust him to touch my own code.
The list goes on ....
Performance Reviews (As asked by Kent)
All of this time, it seems Tim always mimicked my own performance reviews since he always asked my advice on what to write (and although lately I haven't answered him, it seems he just writes down what I've said and done with customers). All my manager knows is that we always go meet with customers together, not that I'm the only one who participates in those meetings, and that this guy is completely useless.
Our old manager, who resigned 8 months ago, might have had some ideas as to how inept Tim is, but I have no idea what he may have told our new manager when he handed over his responsibilities. One telling point is that he always assigns new projects to me, not ever to him.
Current Situation
I am going to tender my resignation to my manager early next week.
Currently we are involved with 2 full SDLC projects which I've been building by myself. After I created the proposals I asked my Tim to create the mock-ups. His work was very, very bad, and made it clear that I can't depend on him for anything. I believe the outcome of these projects will have a very negative impact on all parties involved once I leave.
Is it ethical and professional if I tell my manager the truth about my colleague?
I don't want to burn any bridges with my manager, his aid, or his higher-ups (by leaving the projects to fail). I'm A-OK with ending my relationship with Tim.
I feel that the current situation is somewhat my responsibility, because I have never raised the alarm concerning this guy before. In a way I facilitated his behavior.