Due to the 'old' law by Napoleon, he did NOT want an casino within 1 hours drive by horse-cart... the only real casino near Paris is Enghien-les-bains.
It is easy to reach via RER (like a big metro) open late afternoon till early morning. Like said in a mail in this thread go in suit and tie (they rent you one if needed). Minimum on the roulette is 25FF but they have love denominations at La Boule.
In Paris itself there are cardrooms but do not know much about them... maybe some other board readers do.
Anyway good luck on his trip
Bob