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.
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.
You are a landlord, treat it like the business expense it is and hire a contractor.
In that case it’s even better. Check your jurisdiction, but you might be able to get a contractor to fix it then charge your landlord.