Trust your gut instinct on this one - don't do the job. You meet enough people each an every day to have developed a very good instinct for this kind of thing & those alarm bells won't be ringing for nothing!Just tell her that after reviewing your bookings for the next few weeks you can't actually squeeze her in at all. Apologise profusely - recommend competition (someone you don't like!) and walk away.
No still don't understand, I must be thick