Note, the question makes perfect sense without the link, so How should we deal with questions that have broken links? does not answer my question since it is about questions where the link is needed for the question to make sense.
If the link is broken and the question makes perfect sense without it, then there's no reason for it to be kept. Just edit out the link. It may also help to leave a comment notifying the OP that the link is broken and that you've edited it out.
If you found a web page that you think is relevant and could be used to replace the link, then first edit out the broken link, then comment to notify the author that you've found a possible replacement link. In these cases, you should not edit in the link unless you are completely, totally, 100% positive that it will fill in for the broken link. If the link is to something different, it could appear to change the meaning of the entire question, which is not what editing is for.