Any tips on getting rid of house flies.

It's near impossible to stop house flies getting in to a home especially with windows open etc. I personally invested in a fly screen gard for a door at the rear, I leave open when it gets very warm when I am home.

Net curtains are also a cheap alternative, allowing light in, air flow and a barrier to insects getting in.

Another cheap alternative is Mint plants on window sils , Flies , insects and spiders hate the smell of it .

Bug zappers work but Ironically attract Flies in , they'll obviously kill Flies already in the home but trying to minimise the chances of them getting in, is the trick.

My sense for your friend is net curtains may be a solution . But crucially, it important to minimise having any food left out uncovered, especially fruits and meats, I know common sense but it's important.