Thanks for all the helpful replies, and my apologies for not getting back to this thread before now. I appreciate all the perspectives on this question, even though I don't have time to answer everything.
I am wondering whether a close friend of a family would employ terms restricted to be used rather only among family members... addressing a man as panie ojcu looks quite strange to me I must admit.
It is also important to know what social level the people, who RoseD is talking about, come from since I should expect that different terms would be used by labourers, educated people, etc.
....EDIT: I have just noticed you are asking about the way a youngster would address a close friend of a family. The factors I mentioned are also important in this case. I am adding some more: the age of the friend and the person who addresses him her, sex of the friend.
Thanks for the welcome, Tom!

The people in question are from a fairly small town, well-educated, but not well off (though it would be helpful to know the differences one would find in a village, between less educated friends in the same position.) The young person is a teenaged girl, and the close friends are adults, a married couple her family is close to.
First of all, if you are a writer, you should express yourself correctly. I think you mean : how should a young one ADDRESS HIMSELF TO an adult who ......
There is nothing wrong grammatically with either my usage or yours, as far as I am aware. But thanks for pointing it out, just in case.
Part of my puzzle is figuring out how to transfer these forms of address to an English-language novel. The young girl can refer to this couple in Polish third person in conversation, but when they are mentioned in the narrative, would it make sense to simply write their names? If so, would that be first and last names, or just last name?
Thanks again for all your kind help! I really appreciate it.