As Matt said - depends on what you have agreed/contracted to do.
If you pre-agreed that the model is responsible for MU, then MU is not your problem or billable time.
But, if you offer MU as well as the shoot, then time spent in MU is billable by the model.
As with most issues like this, it boils down to managing the expectations of all parties. Despite what many say, I strongly feel this is best done through a pre-shoot meeting and contract.