I'm not saying everyone has to go to pure drain-tanking, but demo really doesn't shine until you get felguard (cookie cutter). By the time you can actually use those 8 pts, your regular dots will do more damage and pull aggro from your vw (with or without imp. vw) - IMO it is best to use those points to filling out something you can actually use later on (unless he 1) wants to pay more for respec and 2) wants to be the weakest lock in pvp/instances). If he decides to go full demo, it would at least make sense, but he isn't.
Now, his specific question was regarding grinding - let's compare:
1) Demo: Imp VW - Assuming it can hold aggro (IMO it can't ~ I used it up until 35's), it's high armor and consume shadows let's it be an amazing soloing tool. Cons: it can die from multiple pulls (your health funnel is low lvl, it can't draw aggro from all pulls, etc), it's worthless in pvp, bg, or instances (bosses require max dps in which case vw<dt), and finally, it will take a long time to kill individual mobs (you have to keep your vw aggro > your dps).
2) Affl: Drain Tanking - Assuming you're half-wise intelligent when buying your items, your defense skill will be maxed, you won't have deminished spirit (already have +hp), you'll be doing more dps, and you'll end each fight with high hp (can be put into mana with life-tap) - all of which will let you be ready for any other mobs in the area; you can also easily outlast multiple pulls (dted lock can take on 3 mobs at lvl, and 1 additional mob per lvl discrepancy). It also lets you easily outlast people in pvp/bg ~ and during instances, you will be able to last when you catch aggro from the boss. Cons: you take more item damage (requires repairs ~ more money spent), low armor means that you have to be careful about pulling certain elites with greater dmg than regular lvled mobs.
When you compare pros to cons, IMO drain-tanking is the best way to go if you're going affliction.
c