My friend has a ghost in her house and how did it get there

In reality a ghost in a house is much much more than just the spirit of a deceased person. Spirits haunt homes, land, PEOPLE, artifacts, etc... and there are many reasons and how they do this.

Being that I know nothing of your friend or their home, some of the more popular reasons a spirit will manifest in a home is any traumatic event such as death or murder... if it was a place where vices were dealt with (drug use, pornography, etc)... if the place was the setting of dabbling in the occult or other types of worship.
These are many reasons for a spirit that was already there when they moved. Some spirits attach to people and families... in this case the spirit would have been brought there when they moved. I believe also that many times a spirit will move into a home simply because the spiritual guard in down and it can easily move in.