They should have a litterbox for each cat PLUS one- so 3 cats- 4 litterboxes. I would recommend that she try using a Feliway diffuser (also known as Comfort Zone)- it is a pheromone product that can help cats with inapprop urination. This is a really tough issue to address and your friend has to be dedicated to try to make it work out- boxes should be scooped daily, unscented scoopable litter is recommended, Feliway diffuser in area where the cats are in (may need more than 1 per household- depends on set up) and replaced every 4-6 weeks.
It sounds as though the vet checked a urine sample and did not find any signs of an infection- blood, bacteria, etc and I am assuming there were no signs of diabetes either- urine not concentrated adequately or any glucose in the urine. A standard urinalysis that includes a dip strip and sediment stain will indicate any of the above problems.
Comfort Zone link-
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=13043
Here is a link for a litter additive that has been reported with some success in combination with Feliway.
http://www.preciouscat.com/attract-litter-additivebronly-available-through-your-p-7.html
This might be similiar or the same thing that she is already using per her vets recommendation.
It sounds as this is a behavorial problem- and unfortunately it is a difficult and heartbreaking process to work with- trust me, I know! It won’t be cheap and easy to buy the Feliway, add a litter box, scoop daily and use a good unscented scoopable litter but that is her best shot. Other than as a last alternative of trying the cat on prescription meds- which is not only costly- depending on the meds it is given 1-2 times daily- probably for life if it works. Meds are definitely not an easy answer.
Your friend should also look closely at maybe why the cat avoids the litterbox- does it get ambushed by the other cats- is it in their territory? Does it not like a hooded box? I switched to dog litter boxes (about 3 times the size of a cat box) and that seemed to help. Does the cat dislike a member of the family that is commonly in the litterbox area?