My I ask, if you know, what has caused your friend to be afraid of this happening? Has he been in contact with another soul wanting to attach itself?
For many witches, we have ancestors who were witches(or the like) who often attach themselves for either an extended period of time, indefinitely, or only a short time, to teach what they know, pass on the family knowledge, so to speak. I myself have experienced this. Generally its only for a short time, and if not, I've never experienced any of them attempting to take over.
As the soul that is the rightful owner of the body, you have the right to force them out if they try to take over.
B.
There is no reason to be afraid. His body is his and his alone, no soul can take it from him unless he lets it. Spirits often try to trick us into believing we are powerless compared to them, this is a complete falsity used to weaken us and force us into submission. It is only through the weakening of our faith in ourselves that spirits can hope to subvert us, so long as your friend maintains his belief in himself and his control over his fate, he will be able to repel this invasive soul. Remind him that he will always remain himself and remain in control, this is his body and his life, at the minimum he has a massive home field advantage. He will not be changed unless he allows himself to be, so long as his will and faith is stronger than this invader he will succeed.
I'm sure there's a way to invite a spirit into your body, but I wouldn't recommend it. Walk-ins, I think, contact you first and request your permission, they are spirits that are prepared to inhabit a human body. Other spirits may have never even taken a physical form before, if you granted one control over your body they would have no idea how to use it. Ghosts or spirits of the dead have had human bodies before, so they are more open to it, which is why it's possible for mediums to channel them. If you want to learn how to do such, I recommend practicing with your spirit guide.
