Try to call the number: +48 118 913, or if it doesn't work, then +48 22 118 913. This is the help line giving information on people's phone numbers.
I am not sure if they also give information in English. There used to be a separate number for the information in English: +48 118 811 / +48 22 118 811, but the information about it that I find are over 10 years old, so its operation might have been discontinued.
There is no online-available phone directory in Poland.
And I am afraid that it might be not enough to give the name of the specific person, I think you will also need the town where they live. Due to the personal data protection law.
Another problem might be that they no longer have a landline phone in the era of cell phones.