I'd like to thank my screenwriting friend for what he's done, and here's how. Can I?

If your novel is sold it is extremely unlikely you will have any input into how the film is made in any way.
JK Rowling has sold fifty billion books worldwide and has no say whatsoever in how her babies are produced for the screen.
You can refuse the sale but you'd probably never get it produced.
You could perhaps ask he be a part of the directorial staff and they'd probably give him something important to do, like getting the coffe.

There are some exceptions.
Is your friend Steven Spielberg?
If this is the case then it's possible he may be allowed to direct it, although it's not a given.

If a studio (major or minor) is going to fork over money to option and produce a movie of your novel (that's also assuming you're lucky enough to get it published in the first place), chances are they won't take a chance on an unknown, unproven director (ie, your friend). They'll pick one, and it's their right. Heck, the book could be optioned and still never become a film. It's a fickle industry.

The only way to guarantee your friend is the director is to write up a business plan, get investors, and produce your own independant film with him at the helm. Granted, it is way more complicated than those few steps, not to mention costly and time consuming.

But these are all hypothetical situations. Get that novel finished, edited, polished, and start submitting.

The writer rarely has a say on production. Rarely, not totally impossible. But unless you're John Irving and the director is Steven Spielberg, chances are slim.

You're thinking way ahead of yourself. Finish the book. Get it published. Get an agent. Think about it when you actually cross that bridge.