If she has the booking amount from previous events showing what the client would be charged and a copy of a check showing that she got 50%, then it would be a slam dunk in small claims court with or without a signed contract.
The money is owed to her, no matter what the gentleman says, unless he suddenly cancels all the events (ha, yeah, right..) and even then, odds are he would still owe it.
You can go with a lawyer; it depends upon your region. The more rural areas handle small claims just fine, heavy urban areas don't. If you're in a heavy urban area, find a lawyer.
In the mean time, I would submit a bill for those events she has booked at the time she was fired. It doesn't matter if the event has occurred yet or not, and it should be clear on the invoice what the interest rate would be on past due balances and make it clear that this bill is due and payable now since he's decided to terminate the business relationship.
There is no such thing as a slam dunk in court. I was sued for adverse possession by a party that did not own the property and ended up losing the property. Stay out of court if you can because the judges and the attorneys and the clerks all go to the same Christmas party and, by the way, you aren't invited.
The language evolves. "Slam dunk" now means 'we are telling you it is a no-brainer, but it will turn out exactly the opposite.'