Always get a second opinion when dealing with large sums like this. Get at least 3 diagnoses and repair estimates as quickly as possible to see if the Trane condenser really does need to be replaced. Don't just replace the compressor on a unit this old as the outdoor coil is suspect, also. An 11 year old system undoubtedly used R22 so you may need to switch over to R410. That would probably require a new indoor coil also. Was this included in the price? The price may be good or bad, depending on what he plans to replace and what he intends to replace it with.