Long story short, I performed some consulting work for a friend, for the fruits of which his company paid him money. He then paid me that same value of money in cash to my little LLC, to return the favor. He is now asking me to fill out a W9 so he can have his tax accountant process it and give it back to me, for my taxes I presume.

I intended, from the beginning, to pay taxes on the cash I earned, but here's my two-part question:

1) Is he double dipping by seeking a write-off? In other words, getting paid X by his company but then claiming he had a business expense of that same value by having to pay me?
2) Do I inherit any negative tax/income effects by filling out the W9 for his tax accountant?

This is probably a very basic question but I'm not exactly a tax expert, and it's a new situation for me. TIA for any advice.