The problem with cleaning or treating leather is that it can change the colour so will probably need to be treated all over. As first aid - Wash with warm water & a well wrung cloth to avoid wetting the leather too much, wipe away all traces of urine & allow to dry naturally - no direct heat. Exposure to natural sunlight is a great de-odouriser. Then see how much of a mark is left. There are various leather balsam/cream products wich will feed the leather (to prevent cracking) but they may change the colour slightly.
I would consult the manufacturers to be safe. Then use a shower curtain to cover the sofa to protect it until you can break the habit.
As for the cat - it sounds as if the cat is scent marking the sofa as 'it's' territory. Exclude the cat until all trace of the marking has gone & then the best & fastest solution is to use a pheramone spray (not diffuser). Feliway is the most well known brand. You can get it from your vet (�15 approx.) & it comes with complete instructions. It changes urine (& clawing) marking to facial marking - what the cat does when it rubs it's head against you. Good luck.