Never dealt with something like this, but if the problem seems to be connected to some location maybe create a guardian servitor of this place? Or maybe just create a servitor which would act like a bodyguard of your friend?
Anyway, firstly you must ask yourself whether actually doing anything is a good idea. Of course, if you succeed you will be more powerful and you will get experience, but your friend will be as weak as she is now, even more so, because she will be dependent on you for protection from now on and not on herself. You know, if she endures or destroys the problem herself she will certainly be stronger.
On the other hand, I wouldn't ever hesitate to protect and help people I care about or those generally struggling in their lives. Just something I do even against my better judgment out of empathy which I can't seem to control. Sometimes it feels really beyond me.
So, anyway a few ideas I might have
If you can into astral projection then surround yourself with protective auras, spells, whatever-your-pleasure-is and go into her house and do a banishing, alternatively just scout and see what it is you are dealing with.
If you can't, then maybe use Tarot (or any divination system) to get information about what you are dealing with. I'd do the reading myself, but I'm not yet very pro at Tarot, nor do I know your friend and I think currently I could only do readings for myself or for someone emotionally close to me.
Guardian servitor/bodyguard - I used them with great effect, although never for something this serious, but maybe it's just a case of pumping into them enough energy and they will be able to deal with anything? You could even power it up along with your friend, just create a sigil, either from a name of your servitor or from the "statement of intent" - i.e. it's programmed function and give it to her along with instructions how to power it. You know the procedure of creating servitors?
Also, something that came into my mind - maybe she does it herself? I mean most cases of "poltergeists" are, supposedly, people unknowingly thoughtforming their fears and emotions. You said that her father is an alcoholic, she surely doesn't have an easy life. Combine it with some latent magical ability and maybe you got yourself whatever you are dealing with.
Take care, if you have any further questions I'd be happy to answer.
Maybe you subconsciously don't want to hurt them, even if it is a "dream person", and it results in an inner conflict of sorts? Also, dream persons are part of yourself after all.