First, I'd nail down a word count. Depending on formatting, 100 pages could be anywhere from about 15,000 words to 40,000--those are very different books.
Then, I'd make an assessment to see whether or not the length seemed realistic in view of the story the person wanted told. 100 pages isn't much for a biography, unless it's heavily focused on one area of life.
Assuming something in the 25,000 word range, the answer would still depend entirely on the process. I've written books entirely from existing articles, videos, etc. that were quite efficient (say, 30,000 words in about 100 hours, including a revision draft) and others that required extensive interviewing and took three times as long.
When you're writing a biography and working with the subject, you should build in time for a lot of aimless storytelling--that's how you'll get to your best material, but it's rarely an efficient process.
If none of these issues occurred to you, you may seriously want to question whether you are prepared to plan and project manage a book.
