It not the 8 speed cassette what caused the chain drag, since M900 XTR is 8 speed), it is the move to a compact drive front (the modern deore chainset) and then not removing enough links. given it's using a power link (so also a new chain ?) that should be easy to fix by removing a few links.
See if your mate still has any of the old parts lying around that have been replaced
ahhh, makes sense!
My mate got it from another mate who just had the bike and no spares Bit gutting, only having the chainset missing, will have to keep my eyes open