My friend has just got a little dog. He is a cairn/chihuahua cross about 5 months. She said if she doesn't get to him in time the he wee's in the same place, out side her sisters bedroom door. Is this territoy marking? I don't really know about these things so I said I would ask the question for her.

Any help?

Can't really give you a lot of help only that my bitch used to do this.  She would go to the toilet in my sons bedroom even though she had only just gone out to the toilet.  We had to stop her going upstairs and when she did go to the toilet in my boys room we had to scrub the carpet and scrub the floor boards using a disinfectant designed for dogs which you should be able to get from any supermarket.  She hasn't done it since we have scrubbed the room. 

Like I say it's not very helpful and I'm sure someone else will be along to give better advice.

If it's always in the same place that he does this then you could try cleaning with bio washing powder and wipe over with surgical spirit once the area is dry. Always try to catch him in the act so you can correct him, then he will know that it's wrong to pee in that place. You can also buy stuff from the pet shop that you can apply the area to deter him from marking. Black pepper sprinkled around will also help to deter him.
Finally try not to give him the opportunity to mark inside, maybe a ban on upstairs- use baby gate if necessary.