I agree with the other posts - it's hard to give an honest answer without knowing how old the kids are and how close your relationship is with this "friend".
Of course, if you say anything to HER you risk offending her and losing her friendship. If she's a close enough friend, you should know her well enough to be able to say how YOU feel about the children's behavior without directly attacking her or her child. I think this goes best by starting with an example of another child (let's say at a playground) who was hitting and the mother wasn't "protecting" the other kids. Talk about how you struggle to teach your children manners and what's appropriate when other children aren't behaving. See how she reacts? Does she agree? Does she say, "Boys will be boys." I think then you will know how to approach your problem with her parenting style/son's behavior. It leads into a discussion if you think she's receptive.
I think you have every right to say something to the child when he is at your house. If it's your house, it's your rules. In front of his Mom, tell him "we don't talk that way" or "we don't push/hit people in this house". I think this is another subtle way to let him and your friend know that you will protect your children and enforce some rules.
Now if you're at their house or out in public, unless he's hurting your children, I don't think that you have the right to parent him. I have been bold enough when I see a "friend" of my child acting or doing something inappropriate, and when my child gives me that look like "mom can I do that too" or "what's up with that", I simply tell MY CHILD "we don't do that" or "mind yourself". I find that more often than not, the other parent hears me and corrects their child. Believe it or not, one time at my brother's house at his dinner table when his son was throwing food and misbehaving my daughter started (they were 3). I corrected my daughter by warning her that that was inappropriate and she would lose her dinner if she kept acting that way. My brother quickly told his son to stop it because he was getting my daughter in trouble. I said, "No no. I'm only telling her what not to do. You go ahead and continue however you were. I'm letting her know it's not ok." I think leading by example is the best way versus causing a confrontation...afterall, I'm not saying my parenting style is best, just that it works for us.
So my advice, is if you see her son hitting, pushing, then tell him, "Please keep your hands to yourself." And if he continues to say inappropriate things I don't think it's out of line to ASK your friend to help your child understand that he is teaching/modelling behavior for your younger child and that you don't think that some of the words he's using are appropriate for your young child to hear. Again, you're not attacking her or her child.