I too wanted to search for all forum activity by some user, and I appreciate the forums, so let me point out these bugs:

METHOD 1 (gives error: "Unknown table 'c' in where clause")
Go to the forum search page.
Search with keyword or username.
(Doesn't matter what you put in the keywords field)
Enter a name in the "Filter by author" field.
Search post replies
search now.

METHOD 2 (seems to freeze whatever script is executed)
Go to the forum search page.
Search with username alone.
Enter a name in the "Filter by author" field.
Search post replies.
search now.