Sealing a basic hole is a straightforward job that doesn’t require $100 - $400 for a contractor to do. His business sounds like it has one employee (himself) and sending that employee to complete basic jobs is quite logical. While a contractor could certainly get the job done, I have seen my fair share of contractors that are actively bad at their jobs and will produce a worse result than researching the issue and solving it in-house.
If he messes it up based on internet research and burns the building down, or causes some other harm to a tenant then he’s going to have a lot harder time dealing with the insurance claim than if he hired someone whose job it is to do that kind of thing.
As a business expense the contractor price is tax deductible as well.
Wrong foam insulation resulting in overheating or toxic/ flammable off-gassing, wrong amount of foam, attaching something conductive to the copper pipes, accidentally nicking a wire in the wall, I don’t know about a billion things could go wrong, and you don’t want it to be your fault if it does.
"this is for a rental unit so I'm trying to keep cost low"
This sounds like the kind of BS that gives you apartments that are infested by bugs and still somehow charge 4k a month.
He says there are bugs already. Just lucky it's silverfish and not cockroaches. Do the job correctly and it will save money in long term.
Imo, duct tape the shit out of that. I had a similar hole in my apt, and I just did maybe paper over it and then long strips down, layering them a bit, and then over, and it all gets a bit fucked in the middle, but point being you can get a seal. Cost is a half a roll of duct tape