The general rule is that a person is free to do business with whomever he or she chooses. So, in the absence of a written agreement between your friend and the other owner, customers are free to hire your new company.
Here are 2 things to be careful of, though.
1. I'm talking about NEW
plumbing business. If a customer had a contract
with the old company and the customer broke the contract to come to your company, your new company could face some civil liability. Likewise if there were no contract but the other company was in the middle of a job when the customer suddenly decided to fire the old company and use the new company, the old company may have a cause of action
against your friend and the new company.
2. You state that your friend "works" for a new company, not "worked" for a new company. If you are stating that he is a going to continue to work for the old company while you get this new business off the ground, that could be a problem for your friend that extends beyond getting new customers.
I hope this answers your question. If there is anything you do not understand, please let me know and I will be happy to elaborate.
In the meantime, please click "Accept"
so that I may paid for my answer and any subsequent follow ups with you.