Why does 40k mile SAAB 9-5 need a new head?

No heat in a 9-5 the fix for this poor person they needed a ton of work! worst case scenario for no heat! Customer bought a low mileage 40k mile 9-5 and reports no heat... flushed coolant system and new heater core helped for a few days then BAM no heat, we checked everything you could think of and figured it need a headgasket... come to find out a Clogged head, melted valves, warped head, new heater core plus a full flush once again was need....a clogged coolant system this was fixed with new a head and few things. THE ISSUE: We believe someone used stop leak and it got hard and clogged the system or it was a mix of coolants causing a chemical reaction and solidified in the system, or hard water build up could of also contributed to this, whoever owned this car first did something that caused the new owner a horrifying problem.