I'm a handyman, not a roofer. I recently was hired to reroof a patio cover attached to the back of a house. While doing the job, I discovered the main house has nearly 2 inches of roofing material on it. I can't get an exact count, but I'd say it had at least 10 layers of roofing on it. The last time it was roofed was over 5 years ago.
I know this is not good. It probably doesn't help that part of the house has vaulted ceilings, which means there is no support for the rafters except at the walls and at the top. I can see the boards that support the roofing material have a slight sag between each rafter.
I want to know how much "fear" to instill in the home owner as to his current situation. If his roof is in danger of failing, I want to be sure he understands this.
There is almost no way that there could be 10 layers on a roof without severe structural damage. It would be noticeable. Most I have encountered is 6 and there was definate sag. A common mistake in determining the total layers is usually due to the fact that most count the starter strip that is layed for the first layer. So when it looks like 2 layers it is really one. If you count 10, it is probably 5.
IF there is 10....then YES his roof is and has been in danger for quite some time. Good luck with the job. Hopefully you charged enough.