Your friend will have to hire an attorney. They can expect a drawn-out lawsuit for breach of contract. That may go to mandatory arbitration depending on jurisdiction and amount.

Contracts only document the original agreement. In case of dispute you have to get a judge or arbitrator to rule in your favor, and even then you only get a judgment.

Freelancers & consultants: don’t put yourself in a situation where your customer can owe you a lot of money. Customers: don’t put yourself in a situation where a consultant can owe you a lot of work. Structure consulting contracts with frequent defined deliverables and payments.