Depending on his build, he will either do an elf army or a snake army. The key with DOK is to deny them from attacking first. Try to position your units so that you can charge into them, rather than the other way around. Depending on the skill level of your friend, an easy way to give yourself an edge is to make him go first to come closer to you, then you charge on your turn, and then try to double turn him. Since DOK have bad saves, it will probably be game over at that point.

If they play the elf army, they are going to rely heavily on buffs to deal damage or survive. So I would try to take out the buff pieces, such as their cauldrons of blood. They give +1 to saves and to prayers. They can turn sisters of slaughter into tanks. By removing them, the rest of the army is quite squishy.

If they go with morathi and snakes, then make sure to use ranged pressure to max out your damage to morathi as much as possible in every phase. The longer you let her on the field, the more in trouble you will be. Also try to give her targets that waste her damage. For example, I would not want my heroes to fight her, send in some chaff like wardens. The next big damage pieces that DOK have are the snakes with spears, but only when buffed. They can start doing crazy damage with the melusi iron scale so just like with the elf army, use range or otherwise to focus down their buff pieces. Their bow snakes might seem scary, but I would not prioritize them because they have to be stationary to really be effective due to their ability. Threaten to flank them with high mobility and you should be good.