It takes a heck of a flea burden to make an adult cat ill...
If they're not going out, they don't need routine flea/worm treatment - the sensible thing to do is to flea treat the property (all rooms of his flat) properly using a vet recommended product that lasts 6 to 12 months and this will take care of any travelling fleas from other flats or visitors.
The only exception here is - if the flat is SO filthy there is a mouse or rat issue and the cats are killing those and in particular killing and consuming then yes, flea/worm treatment would be sensible.
Even then, NEVER use over the counter stuff to flea/worm, particularly if the cat is sick, its a really easy way to kill a cat. You need to use vet stuff only, and that has to be prescribed by a vet who has seen the cat within the last 6 months.
I never flea/worm unless I see reason to (ie, evidence of fleas/worms in the property or the animal), theres no good reason to stuff animals full of pretty toxic chemicals 'just in case'.
The inappropriate weeing will be a combination of a lack of space, dirty litter trays improperly spaced and not cleaned often enough (they may also have the 'wrong' litter in them, cats can be very particular about what litter they'll use) and the lack of enrichment.
Neutering would be sensible, it would reduce stress by stopping them coming into season and wanting something they can never get.
SO yes this is neglectful and abusive - but your chances of getting your friend to see this and change what they do are low, and as the cats are fed, watered and sheltered your chances of getting the RSPCA or anyone else to take action are also pretty low.