You pronounce Riza the same way you pronounce any other name; whichever way feels most natural to you the speaker. If the person with that name wants you to pronounce it another way, they'll tell you (and if they aren't in earshot, then it doesn't matter.) Pronunciations are clarified using this simple social protocol.
About half the people I encounter pronounce my last name "wrong", I correct them if I'm in the mood to care, and that's it.