What action can a person take to collect money owed for repair work done based on a verbal agreement in state of CA

Customer did not pay for work done.: My husband does odd jobs to make a little extra money. He had a verbal agreement with a friend of a friend to paint the inside of their house. He gave them a quote of $2,700.00. 3/4 of the way finished they decided that they wanted to finish the job themselves. 2 days later my husband leaves a billing statement in the amount of $825.00 (which is considerably cheaper than what he should have billed for. A week later he is told that they did not receive the statement. And they ask if he could leave another one. He leaves them another one. But hears nothing back. He tries calling numerous time but to no avail. He does not have a written contract nor a contractors license which the other party knew in advance before they agreed to hire him for the job and give him a $300.00 deposit. My question is since there was no signed contract what can my husband due to collect this debit