Can you search someone´s Facebook wall for comments by a specific "friend"?

Nope, Search is only for People.

The only thing you could do is expand their wall and do a find on page! Ctl+F

I tried the "look for on this page" tool under "edit" on the "file, edit, see, favorites" etc. bar but it didn´t work. What do you mean by expand? Maybe that was my mistake.

If you scroll down someone's page you will hit an option to view more posts. That is what I meant by expanding someone's page!

To find on page all you have to do is hit Control + F this will bring up a search box to search on the page!

Actually, you can... kind of. Click the "See Friendship" link. This will show the links that bind the two people (the person who left that particular comment, and the person who owns the wall they commented on). It will list all the comments on each other's walls.

Another way to do this is to substitute the ID numbers into the link below.

http://www.facebook.com/profile.php?id=######&and=######&ref=nf

When you view a person's profile, there will be an ID number. Substitute the ID number of each person in for the pound signs (#) above, and you should get a page relating the links between the two.