I'm really tired of seeing the same old "Polanski and Allen are rapists, can we watch their films without becoming rapists?" thing, but I'll take the bait anyway...

The artist and their art are not entirely separate, since the artist is necessary to create the art. It's the big distinction between art and science - if the scientist never makes some discovery, someone in the future will inevitably make it, since the entirety of science is there for us to discover. Art is created uniquely, and no one could make Chinatown as we know it other than Polanski.

However the art itself can be evaluated entirely on its own. I've always wondered about how different our view of some works of art would be if we never knew anything about where a piece of art came from - if all actors only acted in one movie so we wouldn't be distracted by their celebrity status, if we never knew when or where a painting was made, you get the idea. I'm a purist in the sense that when we evaluate a work of art, it's best to consider only the qualities of the work of art itself. I believe that leads to the clearest judgement - a work of art can be controversial, ambitious, be heavy with social commentary on its release, but all those things fade with time, while the work of art stays.

I think my point of view is pretty clear - whatever dubious things Polanski or Allen or Gibson or anyone else did, their actions are entirely separate from the films they made. In your place, I would ask your friend about why exactly he refuses to watch their films - if he doesn't want to buy them because he doesn't want to support Polanski or Allen, that's understandable and there's ways around that. If it's an issue in your friend's mind where he's too distracted by those facts to enjoy the films, then that's just his subjective view of the films. The obvious flaw with this is the issue of information available to the public - let's say your friend's favourite filmmaker goes on a killing spree. Does that instantly change how he views the films? Or, what if his favourite filmmaker was a rapist, but your friend would never find out about this? I think your friend's answers to these questions would be interesting to hear. It just goes back to my previous point - that we should only judge art by information that's given within the work of art itself. Any extra stuff is just clouding our judgement, IMO.

Note: For some reason I assumed your friend is a "he", sorry. So much for making good judgements...