Put the following into your address bar (change USERNAME to the user in question):
This method does not seem to work for questions posted in the past few days or so, but should be fine for an older question
Go to their profile. On the right side is a box that says "Search content by [USER]" If you remember at least one word in the question put that in the search box and hit the search button. The next page gives you the option to "Scope your search..." You can click on the "Questions" tab to restrict your search to questions only, if you need to.
Go to their profile at the bottom is their activity. Click on the show all activity. I don't know of any way to filter by type of activity, but a green square with a question mark icon will be next to all the questions that they have asked, so visually it should be pretty quick to scan their pages of activity.