There is a good chance that receiver will NOT pass HDR content and other issues with 4K video as well.
The best hope is to disable any video processing the Marantz is doing (Make sure "Video Conversion" is set to OFF), turning it into little more than a HDMI switch (with audio extraction). And see what happens.

Unfortunately if he wants the full 4K experience, he might have to upgrade the receiver. Just have to test to find out.

Note: There have been a lot of issues with 4K and the new Apple 4K TV box as well. So may be hard to figure out which is a Marantz issue and what might be an Apple 4K issue, if things don't go well. If the Apple is his only 4K source, I wouldn't have the customer put to much effort into this right now. CES is just around the corner with all the new gear for next year. Including the latest bug fixes for things.