Tell your dad not to be afraid of testing with a cooling system pressure gauge.
Whenever I notice the coolant level in the overflow bottle isn’t dropping as the engine cools off I know it’s time to test & find out why.
The last 2 times that I borrowed the gauge from Autozone I found that the radiator cap would not hold pressure. Replaced the cap and problem solved. Since I had the gauge anyway I also tested to make sure the cooling system would hold pressure & it did.
Other auto parts stores probably also have a tool loan program since it helps them to sell parts. If Autozone isn’t nearby call around.
If dad doesn’t want to do this very simple test, you can do it yourself. Just be sure the engine is stone cold before removing the radiator cap.
If all else fails-as mentioned- just replace the cap and see what happens.