Spraying cat

Well, I am Petsitting for a family with 3 male Indoor/outdoor kittys,and a Female Lab. And I have Petsat here many times! Well Last time we were here, one of the kittys peed on our pillows, and my laptop bag.. But I Didnt know who it was, or if it was marking or what.. so I never said anything to them. Well we are back again,and this time for 11 nights! Well on the first night..*last night* I watched one of there kittys mark the couch out stuff was sitting by. I was SHOCKED! I wouldnt have ever thought it was this one.. he is the most easy going kitty,and the one the other two pick on. It has always been like he was on this earth to eat, and potty,thats it. All the kittys are neutered. Well I dunno what to do about it. I Emailed the owners, and told them he did this,and I wasnt sure what to do.. If they wanted him to the vet or what. Well he called this AM, and informed me that he has been doing that, and they have had him to the vet and even went as far as giving him Female hormones.. But nothing is working.. (But of course, they Never have told me this, I had NOOOO IDEA) Is there anything I can do, to keep him from doing it, at least while I am here? It just happened to be this time, he and I brought our home puters here,since we will be here such a long time.. and he works from home somtimes.. And I Really don't want to have a cat mark on our machines.. *Towers on floor, Not enough room on table*.

Well I am Hoping someone might have suggestions. I Dont know how old they are either.
Please, I hope there is something I can do.

thanks

The only thing I can think of is, how many litter boxes do they have? Perhaps you could add another one or two and see if that helps.

Hmm there is only one. There used to be two.. When I first started Petsitting for them.. But, it isnt here anymore.. hmm, I dont have anySpare boxes at the moment..Wait.. I might.. Maybe I can bring a spare over,and see if it helps. I am Hoping he wont do it again.. *sigh* It smells awful!