Should I trust other people’s opinions, or only the stakeholder?

I self-learning UX/UI right now. My friend asked me to improve interface in his application that will be used to organize every city event. The stakeholders are city officials who will be deciding on buying, however, they are busy people, so the product owner has forbidden me to interview them.

To get around this, my friend offered to answer my questions on their behalf. Though he developed the application and has been in contact with them, I don't feel like it is good idea to trust only his opinions.

Would it be better if I ask somebody who works with the stakeholders to complete the interview, or would it be sufficient to trust my friend?