I didn't notice any of the instruments clashing with each other at all. Also, while the volume levels may be a little wonky between the elements at time, I'm going to guess that's because of the missing vocals. Your composition is really good on this one.
If your friend's vocals don't work, I'd suggest throwing a synth in that has an option to make it slide inbetween notes, and do it to the same tune the vocals were meant to be.