I know frames are the bain of web existence but I have a friend with a frame built site that quit working for him recently. Long story short, his pages that are supposed to load inside the main content frame aren't working. One such header looks like this:
if they fill their page with links to their site, they will have a bona fide URL on your site with a load of backlinks pointing straight to them. it wouldn't be a problem getting it indexed. all they would have to do is link to it from somewhere else.