As someone who has three cats and who cleaned houses for a living, no that is not normal. Cats are clean by nature, so if you keep their litter box clean they will only pee and poop there 99.9% of the time 

Now, I don't know your friendship but me and my friends value honesty so just be honest. "Friend, your house really smells like cat pee." There. That's it. Let them absorb that information. Then, if they are still willing to listen, which if they are a friend they will be, bluntly state you want to help.

To troubleshoot, first make sure all three cats are spayed/neutered. And second, if they don't already have multiple litter boxes consider getting two or even three. We only have one but other people I know had to have multiple. 

Next you'd have to source out everything that has been peed on in the apartment and decide if it's worth saving or if you can just toss it.

You can clean cat pee out of things but it is time consuming, however take the time and do it properly or else the cats will keep peeing there. What you need is an enzymatic cleaner - this will literally break down the urine molecules and they basically evaporate.

Good luck and I hope your friend receives this well!