The neighbor isn't your friend, so...
Just let your friend know they didn't pay full price so zip-it when he talks to the neighbor.
I can relate to dealing with existing piers where yours needs to go. On the deck I had to dig a few up. It's a good thing they weren't proper footers or it would have been a lot more work. The old ones were the size a posthole digger makes and less than 24". A proper one would be 12" x 12" (not a 12" circle) and 30" deep for our region. I don't pull permits but a build to code
This picture shows the top of a new pier and an old one I dug up
On the topic of money for jobs, I did this builder's siding and exterior trim for 10 years. His pay got slower and slower. The short story is he went belly up and I lost money. This house is one I never got paid for. What's really odd is I got a call for some work through a referral from the local hardware/home center 25 years later. It was this house needing gutter work and it was the original owner who I remembered talking to way back then