1998 Volvo S70 Head Gasket problems

I am in the process of buying a 1998 Volvo S70 from a friend. He has kept the car in great shape and kept good records. Nine months ago he took the car to his mechanic because coolant was leaking and the engine was running hot. He was told the car needed a new head gasket, so he paid to have the repair done (plus a new timing belt). The car continued to leak coolant and run hot, so he brought it back to the mechanic quickly. The mechanic said the car needed a new heating core, too. My friend didn't have the money so he left the car with the mechanic until he could decide what to do.

Last week, I agreed to buy the car and have the heating core repaired (yes, it stayed with the mechanic for a long time), but I took it to my own mechanic because I have used him for years and his price was significantly lower. My mechanic said it definitely needed the heating core repair and made it, but said the head gasket needed to be replaced as well. The coolant was still leaking quickly. Also, the check engine light was on and he was getting an error code that indicates a misfire in cylinder 2.

At the time, I didn't know my friend had just had the head gasket replaced, and since it is an expensive repair, I decided to get a second opinion. This time from a Volvo specialist (I found one with good reviews on Yelp).

The Volvo mechanic also agreed that it needed a new head gasket, but while he was doing the diagnostic, I found out that not only had my friend had the head gasket replaced already, but the mechanic who I hired to do the diagnostic did the original repair!

When I told him (the Volvo mechanic) I wanted to check with my friend because he said he had just had the head gasket replaced, the mechanic laughed and said there was no way it had been recently replaced. I wanted to make sure I heard right, so I said it had possibly been close to a year, but the car had hardly been driven in that time (because it was sitting in his shop. He still didn't know the connection) and he said it didn't appear the repair had actually been made.

When my friend went to talk to the mechanic, he said the repair had a 6 month/6,000 mile warranty (whichever comes first). It is nowhere near 6,000 (or even 1,000) miles, but it has been longer than 6 months because it hasn't been driven because of the coolant problem (that was supposed to have been fixed by replacing the head gasket the first time). My friend was trying to decide whether to sink more money in for the heating core, but didn't want to drive it and cause more damage.

The mechanic says he will give us a discount on replacing the head gasket again ($650), but it is still going to be very expensive and there is a possibility he will find more damage from the blown head gasket that will add more expense.

2 questions:

1. My friend originally brought the car in for leaking coolant/overheating engine. That could have been caused by both the leaking heating core and the blown head gasket. The mechanic diagnosed the head gasket but missed the heating core problem. My friend drove the car after the original head gasket repair trusting that the problem was fixed. Once the radiator light came on again (within days of the repair), he brought it back to the mechanic right away. Shouldn't the mechanic have diagnosed the heating core problem before he sent my friend off with the car? And, is it possible the reason it needs another new head gasket is because the coolant leaking problem wasn't fixed with the first repair, so the engine got too hot?

2. It has technically been more than 6 months since the head gasket repair was made, but the car definitely hasn't had 6 months of driving time and the mechanic knows this because he has had the car all this time. It also hasn't been driven many miles since the repair. I know "technically" the mechanic is right, it has been 6 months, but does it matter at all that the car hasn't been driven and has been with the mechanic?