I just got a new client for my accounting practice. The client had fired his previous accountant and there is a large amount of clean up work that I need to perform. Can this large expense still be claimed by the client in the prior years?

Although these expenses may appear to be excessive, they are likely ordinary and necessary business expenses as per Internal Revenue Code Section 162. It appears that the prior accountant was negligent and thus the reason for the large expense to clean-up past tax returns. Please note though that the expense is deductible when incurred or in the current year. They would not be deductible in prior years.