- click on advanced search
- select "Search for single content type" tab near the upper left
- have "posts" selected for search type
- put your keyword or phrase in the keyword box, i do it in quotes just to be safe or because i've always done it that way. phrases will be harder than specific words you think someone said
- next field, put the username, in quotes of the person you are looking up, select the "find latest posts by user" option in the drop down and make sure to check off "use exact name"
then search.
The search feature can be finicky sometimes and usually it works best if you know just exactly what you are hunting for.
If this doesn't work, google caches a lot of stuff, so if it's a very unique phrase that could work too