Please be aware that some areas require licensed installers now - even for low voltage (less than 50 volts) installations. Including telephones, networking, home theaters/audio systems. Or at least some inspections thereof...
If licensing is required in any manner and you/your friend are not licensed then stop.
That said, what kind of business? Any specific services or products? Seems not.
In any case, it appears that you and your friend need to be ready for a steep learning curve.
There are all too many variables involved and starting at the ground level with zero knowledge and experience is not likely to end well for anyone.
However, many entrepreneurs have gotten started in much the same manner and at least survived if not succeeded....
Gimli: "Certainty of death. Small chance of success. What are we waiting for?"
The Lord of the Rings: The Return of the King
So the following is not an endorsement nor an approval of any sort. Just a simple answer to help you at least have some chance.....
You must document what the customer wants and is expecting. In detail with written specifications, requirements, drawings, equipment lists (make & models, standards). Take photographs, measure the barn, check power sources and outlets.
Start looking at speaker and amplifier products. Find the User Manuals/Guides and installation requirements.
Sketch out the barn and start planning the wire runs and connections.
Google "how to set up a speaker system for home". Or use other similar wording and phrasing. The more specific the better.
And look for amplifier and speaker information such as the following:
There are many sources of information to be found. Many are just disguised advertisements so be sure that the information is truly relevant to you and your customer's needs.
"Affordable" is not very helpful: what budget do you have? Either with respect to the contract in place (if any) and/or what the customer may be willing to spend.
End recommendation: Reconsider and bow out of the job. Do work that you know how to do.
[Edit for typo.]