Has nothing to do with level sync. The OP stated their friend was being kicked from NM parties because of their level. You do not need to sync your level to share exp for NMs.
Since an entire party of level 10s fighting a lvl 19 NM can still achieve gold participation, there is no reason at all in the DESIGN that forces people to only play with high levels for NMs. It is purely player choice, not a flaw in Eureka's design.
Now, grinding on mobs for exp, then yes, the flaw is with Eureka's design because you need to be with a party your level or you will not get exp. However, the OP stated NM train.