Hmm...I've been thinking about this. I really agree with pretty much everything everybody else has already said. I definitely believe in the possibility of an evil, or "dark", entity trying to interfere with someone. I've had some nasty little encounters myself. In fact I believe I may have a problem a little similar to this recently. I'm not entirely sold on it being a family member, but who knows?
If I were you, or her I would definitely take into consideration that it may be just a wayward spirit. Or, what the witch doctor told her may have instilled a fear that she is projecting onto her own guide as you said. The only way I would know to fix or answer that would be to meditate on it, or possibly try to contact the spirit somehow. If you (Doug) can contact your spirit guide, perhaps she can shed some light on the situation? I don't really know. Sorry, I know that's not very helpful.
Assuming there is an evil entity latched onto your friend, this is what I would do:
In my own personal experiences (and I know this is going to sound cliche, but it's true) anytime you forcefully demand that a spirit go away they listen. I don't know why that is, I just know that it has always worked for me in the past. I also have had a very trusted friend give me that same advise on my current situation (That I mentioned above). You would think after all my past experiences I would have been able to figure that out, but you know what they say about the outside looking in. So.....pulling myself back on track here, I would say, no matter how trite it sounds, maybe she should address the spirit, or entity, and tell it to leave her alone, and send it on it's way. Like I've said I've had great results with this myself, for whatever reason it always seems to work.
My current problem sounds quite a bit like you are describing. I intend (after some great advice from someone else) to confront it as I have other entities in the past, and tell it to leave me the h*** alone. Also, I plan to do some serious work on my own personal shields and astral protection. I think this would possibly be good advice for your friend also. After all it couldn't hurt anything, and we all need effective shields in place right? (This is what I am currently learning)
A possible reason this is happening to her is that the entity is trying to prevent her from growing spiritually (possibly she has abilities that may be strong, even if maybe it's unknown to her?), or it could just be feeding off of the negative energy it is causing from her fear. Those are just possibilities though. For what it's worth I do believe things like that happen to people.
With all of that being said, I would definitely try to get a second opinion, just because I think that is always a good idea. Also, I would want to make absolute certain that it definitely is an evil entity because (I know you all have heard me say this before) the power of belief is incredibly strong. If there isn't actually anything, but she truly believes there is, she could create something or draw something to her. I know that sounds out there, but it is my belief. Before I took on that thought process I would want to make darn sure it was right.
Just in case demanding it to go away doesn't work for her (I think it depends a lot on whether she believes it will work), I know some other things that may help. I'm very good with runes and making a bindrune or runescript is an easy and
very effective method of banishment and defense. I would be more than happy to tell you how to do either a spell with runes (I'm very simple and don't use a ton of odd things in my spellwork), or I could tell you how to make a charm or talisman with runes for protection. Heck, I could even make one and mail it to you, but it's very easy to do yourself. In this case I would use Eihwaz, Algiz, and Tiwaz. And if using runescript I would put Ingwaz at the end. I would suggest demanding the spirit to go away and leave her alone and then maybe making a talisman for protection afterwards. For the psychological benefit if nothing else. Of course this is only if you feel it would take something like this to banish the entity and protect her from it in the future, and I'm also assuming you know it is in fact a spirit with ill intent. If worse comes to worse I know a great banishment spell

. I seriously do, but I don't use it unless everything else has failed. Anyway, now I feel incredibly embarrased for saying all of this about spells and runes

But, if it helps it was worth it lol .
I hope everything turns out OK! Please let us know what happens one way or the other!