Definitely do not leave it blank or make something up. Give the most accurate dates (month and year or season and year) and addresses street, city, state or province, and country) as is possible. Your wife could ask family and friends if they remember more info than she does. Maybe they could help recreate the data. For example, My older brother had an uncanny memory for numbers (like every phone number we ever had growing up).
They do not expect everyone to remember everything, but they will wonder about what the applicant is hiding if there are gaps. The addresses will generally correspond to what she has listed for schooling and work during those times in the separate section of the app. So, if she can't remember the exact address they will see she was enrolled in XX school or working for YYY company at the same time.
If she can't get it exactly, I suggest writing something like this.
(street address unknown), Charleston, South Carolina, USA ??/1992 to 07(?)/1993
I'm sure others will suggest how they dealt with this issue too.
Best wishes, Allison