Pioneer DJ controllers aren't really designed to be used with general purpose music players. You can probably get playback to work. I'll detail that in just a moment. But, even if you get playback to work, it's unlikely that any of the other DJ features will work: Play, Cue, jog, tone controls. Certainly the loops and other effects won't work.
I wonder if your customer is expecting DJ functionality or not. If not, I have to wonder why they would want to use this deck with MC?
On to how to make it play: The error you see is almost certainly because the DDJ wants to see 48kHz audio and it is seeing 44.1kHz audio. You can manually set all playback rates to 48kHz using the DSP Studio (Player > DSP Studio > Output format).
Or, more easily, just set MC to auto configure it:
Tools > Options > Audio > Advanced > auto configure settings on playback error > YES
Good luck to you and your customer.
Brian.