Makes sense to me if you think about it, if his site is free (and hopefully it is) his members are paying nothing yet hes paying for server and bandwidth fees, a required click on some banner before a user can download the file would help even out the cost
I can see it as once they click the ad the image would be replaced with the download link, the click would make him money and the client can close the new page.
I never got into using 'ads' to make money but this is an interesting idea.