I am not a professional, but I do not think this is normal behavior or caused by his fathers speration. I think that he should be evaluated by a professional. Bipolar disorder in children often manifests w/sexual acting out and extreme aggression. It can also be a sign of sexual abuse.
The only thing I really want to caution here is that the child is not the bad guy. So often, a new fiance/girlfriend will want to make it out as if . . . only if that kid weren't around, things would be good. This boy is an innocent child that sounds victem to some bad circumstances. He may have some other issues going on but all of this requires empathy for this poor little boy. I hope that the adults in his life show him that.
What is the child's life like when he is not with his father? He sounds like he is being ignored at best, harmed at worst. Is the state involved at all?
Your friend owes it to her children, including the one with whom she is presently pregnant, not to bring them into this situation permanently until some of this is ironed out. It doesn't matter how much she loves the man, the kids are the innocent victims of this adult stupidity (pardon my judgment, but anyone who gets pregnant in a situation like this is putting their own wants and wishes before their duty to their kids). I wouldn't force the kids together at all, and I would tell the man I won't marry him until his son gets the help he clearly needs.
Hm. Well, first let me say that when seperation of parents happens, many children do not handle it very well. It is not at all uncommon for a child to act out. There is a difference between the two households and it causes a feeling of instability. Certain rules at one place an not at another. Different routines, etc. Also, with his mother . . . he most likely does not have new, unknown children taking away from his father's attention as he does when he is with your friend and her kids. That is hard on most children. Some handle it better than others but it is helpful to put yourself in his shoes and picture how this must all feel to him. Adults think kids should just get over it and love every thing they do . . . but it isn't always that easy. And his feelings are somewhat valid. They'll get farther for ackowledging this than just viewing him as horrid.
Also, from your description, I'd say that this boy is not supervised very well. I say that because if he is with a younger child and has time to get them down and choking them until they are blue . . .um, where is the adult? I think that some kids you can wander away from and others you can not. This little boy needs more structued play and supervision. Your friend has her hands full . . .two other kids and a baby on the way. But, when you have that many little ones---------- you just have to make it work. She won't have much time for herself and getting other things done . . .but the kids won't be little forever. I had two boys close in age and I took showers at night while they were sleeping, for example so I didn't have to leave them unsupervised.
He does sound like he has some issues with aggression. Is anyone rough with him? Could he be modeling any type of behavior? I think I'd tell your friend to go to the library and check out books on feelings. In the kid section they should have a high number of books that put the feeling of anger in kids words that he can understand and use. There is also a book "hands are not for hitting" that is good and I would get. I'd make violence an automatic time out. "SOS for Parents" by Lynn Clark goes through the proper steps time out. There is a wise person here that has researched it and says that it takes about 3 weeks for any change in behavior to happen. So keep trying things for a while before you decide it isn't working. Pick one to two things at a time to work on.
Does he get much time with dad alone? The fiance and her kids should not always be a package deal with dad. They need their own time together and to do fun things where he feels like HE Is the most important thing in dad's life. (all kids want to feel this way.)
The rubbing is somewhat a natural thing that some kids stumble upon. I'd make a rule that he can do it but only by himself in his room.
I don't know how this will go down for the long term for this family. A new baby into the mix will make it that much more stressful. Bottom line, the dad has a responsibility to the boy and the relationship should be promoted. How it gets to one happy family, I do not know. I think it will be helpful if your friend tries really hard to bond with him (again, seperate from all of the other kids). He isn't "horrid"------ he's just got some issues to work out. He's a little boy that will learn over time. The adults need to always stay calm with him and keep his best interests in mind. But it will take time and patience. I wish your friend had noticed these issues with her fiance's son before becoming pregnant. Then she could decide if she could handle the stress of his child or not. Now she kind of has to.
Anyway, I've also found getting down on a child's level to speak with them helps and use a calm voice. Make violent, aggressive actions a total no no------- that is an immediate consequence and time out. I use the phrase "hands to self" or "body to self". Try organized play with him to keep him active and involved. Give him choices when you need him to comply. Make him feel welcome and loved. good luck