Legal Question in Real Estate Law in California

You do not tell us who put up the deposit. If your lease forbids having roommates why are you trying to get one? The economy is bad for everyone, including your landlord probably [most owners of rental property depend on other scouces of income to live as they only own 1-2 units]. Once your landlord finds out that you are in violation of the lease by having a roommate [doubles the wear and tear on the unit, increases the utilities, etc.], he/she will probably move to evict you. Even if you should win by showing that either the lease does not prohibit roommates or violates the law in trying to do so, you will have on your record the fact that an unlawful detainer was filed against you and a future landlord doing a detailed credlit check will find that out and refuse to rent to you.

In the future, never agree to anything with any one without a written understanding [which is what a contract is] as to what the duties and responsibilities of everyone is. Just tell the other potential roommate that you are not allowed to have roommates, that he breached the agreement by saying he would not move in, and you can not be sure of his actual intentions so it is best that he not move into the unit. If he gave you any money for the deposit, you must return the full amount as you had no right to make any agreement with him and you suffered no damages by his not moving in.