^^ Is that from a Christian based view? (I'm just asking.)
This answer varies on culture.
Some will say spirits stick around for a period of time after dying, some will say that "impressions" are left in a home(similar to haunting) or on certain objects. Some will also say its just in your head. And some will say as leon did.
There has been no proven fact that any of these exist, and so answers are looks for through your belief system.
Spirits are merely residual energy of a person which has passed on. They are indeed like people. As far as influence goes I would think they could being that of the metaphysical sense. In a lot of beliefs spirits can indeed affect the immediate predicament. Some beliefs hint they can cause sickness and excessive negative energies in the life which can create conflict between those in the household or those which are in your everyday life.
This is why cleansings and banishings have been made in effort to get rid of the influence of the spirit.
So it is not scientifically proven, but magic is not science even though it can go hand-to-hand with one another in quite a few things. If this is a problem with your friend then they should consider learning how to meditate and focus and then move on to learning to banish them from their life.
Most people when they die continue onto other plains, and other experiences. A few are trapped here in this world, usually due to trauma or because they have a task that they feel a need to complete before they move on. After they move on, the dead can also look back to this world and offer advice and help. But most negative influence is usually gone with the loved one. Unless there is reason to suspect that they are still bothering your friend I wouldn't worry about it. If there are weird, unexplained things going on I would just start with a good smudging to clear the negativity and see if that helps. If that doesn't help, I would look into a good cleansing ritual.
Not specifically, I am more of a Deist than a Christian (I'm a Christian Deist actually).
Have been a theistic satanist and I've been told that demons do impersonate the deceased in regards to people who will fall for the act (my info is good)
I asked because a side of my family is very strict in their Christian religion, and they've sworn up and down the same statement. (No offense given here, I was just wondering if this was a similar case.)