My problem is what to do with half an avocado when you haven’t eaten the entire thing.
I don't have any ideas about that; I... just eat the whole thing. You can't persuade your husband to eat the rest? Obviously, adding lemon juice will help them not turn brown for a few hours, but that's it. It's always been said that if you've made guacamole or something, and want to hold it for awhile, you should put the pit back into what you've made while you're waiting, and that will keep the dish from browning. I've tried it, but don't really know if it works. If it does, again, it won't save whatever it is till the next day.
Of course, if you're just saving half, then save the half with the pit, and leave it in. Sprinkle lemon juice on the fruit. But I still doubt it can be saved overnight.
Some stores, including Trader Joe's, sell bags of four small avocados. That might be a solution, but... They tend all to be at the same stage of ripeness, so...