It is hard to know on the mechanic. First the owner may not have heard or listened to what the mechanic has said. Also if the plan was to sell the car, maybe the mechanic chose not to mention it. At 100,000 miles, that is the time to do the change therefore the mechanic might have waited to recommend it (for example, owner maybe only accumulated 2K miles per year), You could ask the mechanic what he thinks about the car if you drive it in, aka when do they recommend it be done. He should say now for this car.