Same price, new product?

Sometimes people surprise me.  Luckily, this isn’t an issue with MY clients (I’ve been very lucky), but a friend of mine is a freelancer here in town working with a variety of silly issues.  His clients book him for blocks of time as well as a specific end product.  Every job last year went a good 2 weeks to a month or more over the designated time period.  He ended up triple booked a few times because of this, and his clients just didn’t understand.  They think a postponed deadline is great!  Well, it would be, except they don’t expect to pay for the extra time because they have booked him by the job.  Because of this, he has had to be a bit more hard-nosed in the bookings this year and not only does the job have an end date (after which an hourly rate applies), it also has stricter rules on the client changing the parameters mid-job or making more rounds of changes than allotted (again an hourly rate or a new price applies).  It sounds great, but now clients are trying to weasel out of this.  They don’t think that the days spent waiting on them count toward the deadline (but it’s days that can’t be booked with other clients because they are booked and this waiting is because the client doesn’t have their act together).  They also try to get out of the revision count by pretending it’s my friend’s fault (despite having approved the project the week before).  It’s insane!  Good thing there’s a signed contract.