I'm gonna say the obvious and go with Assault here. The versatility and movement capabilities IMO are the easiest to get used to when you first start out. Gives him a little more freedom to explore the arenas as well without getting caught up in a fight without being able to escape as often. It's important to know the maps! Even though they are pretty simpe in MNC and doesn't take more than 4-5 rounds in each one to memorize the layouts. But pretty much as long as he's not like every other noob that just got the game and says "OOOOMMMGGGG I WANTZ DUH NIIIINJAAA!" he'll be fine. No offense at all to the great assassins on this forum!
When I told him bout the assassin he was oh that sounds cool I'll try that and I was like no you have not learned the assassin fully
Assault has one of the more preferable default endorsement builds anyways, along with tank but tank is a different story. or this.
I'd tell you but you just tried to melee me. :| Have your friend use a Tank in Blitz until they have enough to unlock a custom class. Shouldn't take long. Then as long as they understand the role of the Assassin, go for it.