I can't imagine the owner winning a lawsuit on this, if his company is big enough to fall under Title 7 rules anyway. Title 7 says plainly that you can't discriminate against employees on the basis of religion. Firing a worker for not participating in a Christian prayer service is plainly discrimination on the basis of religion. However, I don't have any problem imagining Alliance Defending Freedom twisting logic pretzels around the issue to claim the owner is being discriminated against somehow.

That seems to be what they do.