How to reference data someone told me in person?

The conventional way to attribute such information in your references is to cite your source as "So-and-so" (i.e. the person's name) -- "personal communication". Of course you should get the person's permission to reference them, ideally in an email or some other written format, to be sure that there will be no miscommunication about them having given permission for you to reference them and the numbers they told you. HTH.

I don't think you are using the right definition of plagiarism. It generally means to use someone else's ideas, words, or in this case, data, and pass it off as your own.

You can use the data, cite it as a conversation with the financial investor of the building, and then defend or refute the basis of your findings, i.e. what is the equivalent coffee shops at another location compared with yours?

As far as credibility goes. You can find resources that attempt model coffee-shop profitability and see if the numbers your financial investor match up.