My friend and I are having a blast playing this, and the combat feels so good. But he's been facing a problem with disappearing character models in combat.

He's not able to see the enemy models, but he moves his mouse over to their location, he can still interact with them.

If it helps, his system specs are

Windows 8.1 x64,
4 GB RAM,
Radeon HD 8650g GPU,
AMD A-10 5750M Processor