A. The main difference on Passover between Sephardic (Iberian Peninsula and North Africa) and Ashkenasic (from Northern and Eastern Europe) is regarding Kitniyot. Rice, corn and beans are not biblically considered Chametz, but the Ashkenasic Rabbis prohibited them on Passover nearly a thousand years ago. So Sephardic Jews eat rice on Passover while Ashkenasic Jews do not.
Here’s what’s important to remember regarding a shared space. No one thinks that Kitniyot (rice, beans, corn etc) are Chametz. There is no problem with ownership on Passover or having them visibly around the house. So your Jewish roommate can have the rice, eat in the kitchen, there’s no problem with that.
Here’s one issue: There’s some argument as to whether Ashkenasic Jews can eat Passover foods out of the same pots or dishes previously used to cook or eat rice or beans. The consensus seems to be that it would be much more preferable for the Sephardic roommate to have his own special pot for rice and beans and wash that separately – and they can cook and eat everything else together, providing that it is Kosher of course.