You may have had a point if, very shortly after you'd been driving the truck, you noticed it was overheating.
But the truck very clearly went faulty when you had been driving it. You are extremely fortunate to only be picking up half of the bill, because:
You noticed that the truck was overheating and that it was losing water. But you continued to drive the vehicle AFTER you knew there was a problem. Did you think that the problem was going to go away just by topping up the coolant?
Sounds to me that the original problem was caused by lack of coolant being in the cooling system, or that the cylinder head gasket was leaking. If it was the latter then the part would have been very cheap though with high maintenance costs - it's a big job! But by continuing to drive the car and worsen the original fault, you and your husband are directly responsible for warping the cylinder head. Now you need a reconditioned haead plus the cheap gasket that might only have been needed in the first place.
I appreciate that you may be short of cash, but not maintaining a vehicle is a false economy. It costs more financially in the long run. Don't fall out with your family about it - and yes, no offence meant but, you are very clearly wrong.