I don't see the need for any y cord at all. All he has to do is what you've all told him to do and that's to simply pan both of the two stereo channels to either right or left and then send that side all by itself to the speaker. You'll have both signals in a mono form.
Also ..... not specific to the Beri because I DON'T KNOW FOR SURE BUT:
On some equipment .... the separate outputs can have a floating ground and summing two separate outs together can cause hummm ...... or in worst case, as XLR mentioned, damage.