Yes, Darmanitan's health will still go down by half. This is because Sheer Force only takes effect on attacking moves, and Belly Drum is a status move. Sheer Force actually does not work by only removing "positive" effects (for example, it removes Life Orb recoil, which is a negative effect) but instead works on moves with Secondary Effects and removes said Secondary Effects. You can learn more about Sheer Force here.
