Well, if he gave it to you as an .swf file... that makes it more challenging, 'cause then you can't edit the content (ie. add the link), and you would instead have to make the link itself independent of the film.
I don't think there is a way of controlling frames in a Flash movie from
outside the Flash movie... and having the page reload (or relocate) after a fixed time wouldn't work too well - people with a fast connection would be waiting and waiting after the film had finished, while people with a slow connection would be cut off halfway.
If he gave it to you as an .swf file, I suggest you go ask him to give you the .fla file instead
Edit: Duh, how dumb can you get? *slaps own forehead*
You can of course just make a new Flash movie where you load the other .swf file into the first frame and then tell Flash to move on to the next frame (the link frame) when the other movie is done playing (using telltarget? Is it still telltarget? My Flash is getting quite rusty, I haven't really used it in over a year)...