The cats may all be spayed and neutered, but they will still urine-mark and spray (both males and females), often because they are stressed out and unfortunately, as the urine smell gets established on materials and surfaces, they are more likely to keep going back to it. There are many, MANY factors that play into this behavior, including numbers of cats, numbers of litter boxes, the litter box set-up, and other sources of stress (such as people coming and going and household changes). Because it's been going for so long and there are 5 cats, there is no quick easy fix. First step is consulting with their veterinarian and possibly a referral to a behavior specialist. Sometimes anti-anxiety meds can help. However, since these are not really your cats, you need to talk to your friend about this - hopefully she will be willing to reimburse you for veterinary costs if you have to do all the work. Anything that the cats have been peeing on needs to either be thrown out or properly cleaned - sometimes that means replacing carpeting or furniture. There is a product called Nature's Miracle which does a really nice job of getting rid of the urine odor, but again, that's only one step in this whole process.
EDITED TO ADD: They need litter boxes if they spend any amount of time inside but if they are used to going outside, they may not actually use the boxes. Personally (and I very rarely recommend this but sometimes it's the easiest solution), if you say they have all been outdoor cats, let them still be outdoor cats ONLY. I love cats (obviously) and I am all about advocating for their welfare and fostering the human-animal bond, but everyone also has a limit. The damage cats can do urinating all over the place is tremendous, and as stated, there is no quick and simple solution. As long as they have shelter in the garage and access to food and water, and have been acclimated to living outdoors, they can do very well for themselves. But you should have a serious talk with their owner first and see what she is and is not willing to do about the problem.