Heater blowing cold air on passenger side

More then likely this is the blend door on that side. The blend door is responsible for routing the heat over the heater core and if it is not cold then this is the most common cause. Here is a guide that helps with this:

https://www.2carpros.com/articles/replace-blend-door-motor

Below you will the process from the manual on this. Let me know if you have questions. Thanks