We have had a few Ask the Expert questions about underlayment. Here is an example and the response from TRI President and Technical Director, Rick Olson:
If the underlayment is not deteriorated or buckled can you go over it with new underlayment?
A good question. Without seeing the roof, it is hard to know the actual condition of the underlayment. As the secondary barrier for water shedding, care needs to be taken to make sure that the underlayment is in good shape prior to re-installing tile over. The reason is that the tiles will last longer than the house, but the long term performance of the secondary barrier will determine the life cycle of the roofing assembly.
If you are re-roofing and already have the tiles off, it is the time to replace, since there is minimal costs to remove and replace. If the roof underlayment is relatively new and appears to be OK, then yes it is allowed by most code jurisdictions to install an additional layer. If there were problems and you add that layer, you are just covering a potential future issue.
Again, without seeing the roof in question, I can only theorize what the options are. I would check with the local building department about the approval for an additional layer.