When I was pagan, a spirit guide was the cool thing that divided the newbies from the real experts. It was thrilling just to have someone over who had one and especially if s/he could prove it. Then we all got them. They were surprisingly easy to get as it turned out, and then they wouldn’t go away.
They always started sweet and friendly (well, most did). But they mostly turned evil by little steps. We would try to get rid of one that had turned on us by inviting a stronger one:doh2: and wind up harassed by both.
They are sometimes definitely real and dangerous.
If your friend has been talking to her guardian angel one way to find out is as Mystic Warrior suggested. Tell her, if her guide ever wants her to do anything obviously immoral, she has all the info she needs just from that alone, regardless of what the guide says about the reasons.