I totally agree, if the current owner is your client, wouldn't you have to advise him to close the books on the date of sale.
In fact, if the seller was my client I would write a CYA letter.
The other big issue is "conflict of interest." There is no way you can advise both the seller and the buyers.
The differences between an "asset sale" and a "stock" sale how the seller and the buyers will be effected could be quite dramatic!