How old is the other partner?
They are only asking for advertising for their site. If they are underaged, isn’t it up to their affiliate programs and webhost to determine age restrictions?
Either way, I believe it comes down to whether or not you’re comfortable with selling them banner space. I can’t think of any way that you would be held liable for their participation in promoting poker. Even when one could advertise online gambling via Yahoo and Google, there was never any screening of ad buyer age.