Illness override mod?

Hard to say without knowing the exact features of that mod.

There's one here:
https://www.simrealist.com/private-practice
And anoteher one here:
https://modthesims.info/d/656560/health-system-overhaul-illnesses-blood-types-dieting-and-more-v2.html
Also I believe Slice of Life has some illness modules:
https://www.kawaiistaciemods.com/post/slice-of-life-mod

There are also some smaller ones, but judging from your post, these aren't what you are looking for.

@83bienchen I've been through all of those. I stopped using the SOL Mod last year because it was annoying me half the time.

Maybe you could give some more insight then as to what the mod that your friend is using actually does?
Do you maybe even have a screenshot of what it looks like in game?

Btw, I'm totally with you about SOL being annoying half the time - it's not my cup of tea either.

I do have some more:
https://modthesims.info/d/605259/cooler-sicknesses-mod-v-2-2-2.html
https://modthesims.info/d/659888/sickness-gamepack.html
https://adeepindigo.tumblr.com/post/651072235459772416/healthcare-redux

You might also search for life tragedies (I don't think that one is fine to be linked here).

Sorry, wrong thread.

Life's Tragedies isn't what you want. It's not about illnesses at all. Tried it misunderstanding that and it was a huge mistake. Private Practice on the other hand is a great mod, if you can withstand everybody being ill up front, that is. Until they reach 'herd' immunity. I wish I had another mod to recommend, but I'm only using a very limited set of mods.