How frustrating - all it takes is one other parent who is lax about access to inappropriate material on their computer, and concerning appropriate use of language, and suddenly all of the parents of kids who know this one have a problem on their hands.
I think your response needs to be twofold: I agree with you that you should have a conversation (uncomfortable, I sympathize) with this kid's mother. Who knows if she even is aware of what her son is doing and saying? She really needs to know so that she can begin addressing his problem behavior (inappropriate language and looking at adult websites). This area is pretty much up to her. However, there is an area where you have some say and control- your son's awareness and knowledge of sex and sexuality. This is up to you, and you still have a chance to have a positive impact on his perception of girls and women, sexuality, and himself.
It is so important that we as parents teach our children about sex and sexuality; if we do not, choosing to leave them innocent, they become prey to people and situations such as those that your son has encountered. Innocence, sadly, affords no protection for our children. Knowledge does.
I made a decision with my child to introduce these difficult topics starting around age of 5-6. Why so early? because children at that age are old enough to begin to understand the very basic facts, and more importantly, this way I can ensure that my child will learn about sex from me first - not from somebody else (a kid at school, often misinformed), or from something else (an adult web site or magazine). It is inevitable that our children will learn about and be exposed to sex and sexuality earlier than we would like. (Think about how old you were when peers and classmates started talking to each other about sex; 9 years old is when I remember a lot of that starting to happen.) We can't change that fact. We can, however, prepare and protect them as best we can by arming them with knowledge and - as you say so rightly - keeping communication lines open, so that they know they can always talk to us.
To be honest, I had a really hard time - at first - approaching and discussing these issues with my child. But I found some good and helpful books at the library and bookstore which really helped. One in particular "It's not the stork" is a good one for young kids, using appropriate and sensitive language. This way I could read from the book to my child, and then she could read some of it herself and talk to me about it. I think it is very likely that if she hears or sees something inappropriate she would come to me and talk about it, but even more importantly she would have a context - based on what we have already talked about - for confronting and processing what she sees and hears. My hope is that this way she can challenge (mentally or verbally) inappropriate or mis-informative behavior images and languages more competently when I'm not there with her.
This is so hard, and you are very brave and caring to stay open and supportive of your kids. I'd say you also have a golden opportunity to start the conversation with your younger son, perhaps saving him and you some grief later on. Younger siblings inevitably hear things from or through their older siblings, so you may have to start the dialogue with him now anyways.
Good luck!