There must an agreement in placed between your client and your employer company. There must a certain restricted clause to hire an employee from the client within a certain period of time (i.e at least one year period from the termination date). Normally, nobody will do business with client/vendor without signed agreement between two or more parties and placed a NDA [non-disclosure agreement- might be unilateral or bilateral or multilateral]. In case there is there an existing contract, it will then create problem for you- no matter what is your working status either OPT, CPT, H1, GC or U.S. Citizen. Why not request a copy of signed agreement from your client's manager, if he is aware of your situation and go thoroughly it.

There might be a paragraph/wording like:
Company (client) can't hire current or former employees, who again terminate employment, must have completed at least two years of service during the reemployment period to be eligible for prior service if rehired in the future.

Note: It's my two penny information, though I am not a lawyer.