"A mio padre gli piacciono le mele."
That's a bad mistake, please report it!
The correct sentence should be either
"A mio padre piacciono le mele." without "gli"
"gli" = "a lui" in this case, it would be like saying "A mio padre a lui piacciono le mele" and that's wrong.
To understand a bit better the function of "gli" as a particle,
And if you have problems with the verb "piacere" check the FAQ #10 as well.