If everything works fine, no need to change any settings...
If you don't use 5GHz band, you may disable it. But 2.4GHz Wi-Fi and 5GHz Wi-Fi should not affect each other...

Would it help at all in disabling the 5Ghz for the guest cottage, I mean would it improve power or as mentioned above have no effect?
If the signal appears too weak in the other cottage would a repeater/ AP or signal booster be the best solution? Less $$$ is high priority for him. (why I'm stuck with the job).
I've tested the 2.4 signal and in my house it easily reaches across the street from the middle of my house with many walls and a garage.
His cottages have minimal walls and minimal insulation etc...

Thank you for your time.

You'd better test to see if to disable 5GHz helps or not...
You can try different channels like 1 6 or 11 and you can also try channel width 20MHz.
If none of above settings work, you may need a range extender.