You are correct, that the direct translation of "Have you ever watched this movie" is "Hast du jemals diesen Film gesehen?". And your friend is also correct, that usually very few people would express the question that way in Germany - except for in a few circumstances.
I think, this is rather a cultural difference than a linguistic one. In the US in particular, people tend to use hyperbole, exaggeration and extremes (from a German point of view), and you often hear words like "Awesome" or "Fantastic" in everyday conversation. In German (and to a good deal in British English as well), this is not as common, and the conversational tone tends to be more modest - which is considered a good thing and people boasting and exaggerating are usually rather frowned upon.
A similar phenomenon occurs with "Have you ever watched this movie?" In German, this would imply a rather probing question on whether you are actually sure that you have not EVER seen it, where this connotation is probably not as strong in English. I personally would simply ask "Hast du diesen Film gesehen?", without using "jemals".
As to "schon": "Hast du diesen Film schon gesehen?" can actually simply be translated by "Have you watched this movie?": the "schon" is implied through the use of the present perfect in English, and rather refers to something in the not too distant past that has not been completed in the present in this case, and the same feeling is conveyed through "schon". In your example, it would mean "Hast du diesen Film (der gerade ins Kino gekommen ist) schon gesehen?" - which I guess is not really what you were trying to ask.