This is a sticky situation for sure. I'm sorry you're stuck wondering how on earth to handle this, and especially since you know the mother well, live so nearby and aren't sure she'll be responsive.
From my perspective, putting myself in your shoes, I'd mention it to Jane even if it blew up in my face. If her difficult child tries to deflect by blaming your difficult child, that's on her difficult child, not yours. You've looked into this enough to believe it has merit from your son, you've seen his story not waiver and he hasn't back peddled when faced with knowing that you intended to speak to the boys mother about this.
This boys mother may or may not decide to take this seriously. She may say boys are boys. She may deflect by pointing fingers at your difficult child. You really can't know until all is said and done. You certainly are eloquent and intelligent and I know you could, if you decide ultimately to speak to her on this, put it in a non judgemental way. There is a chance that she'll not believe or say the whole boys will be boys thing. Even if she says that boys will be boys, you could respond without criticism of her take on this, that yes indeed boys will be boys. That you thought she should know because while you aren't dealing with this information with a knee jerk reaction, other parents may and if Jack approaches this type of thing with another child of reactive parents, you are concerned for her son that he could find himself in a boat load of trouble. At the minimum, even if the mother does nothing or doesn't believe or whatever, this knowledge might help in the future if another child makes accusations of being approached by this boy. Then the mother will recall what your difficult child said and it might prompt her to take a look at her sons needs.
I find it very sad to think of a child with bipolar not having more help than this woman seems to be giving her son. That does not bode well for the thought that she will deal properly with this situation with her son. But it may be helpful to her in the future as her son is a difficult child, this situation aside. I also think that it is important that when we teach our children to come to us when things like this happen, that they see we follow up and handle it appropriately. In this case since it never escalated into more than talk really, appropriate is letting the boys mother know what was going on. She need not freak out, I mean kids DO explore at that age, and some kids really have no clue about appropriate boundaries and curiousities. And many kids get afraid that a friend will tell and make "threats" such as saying it will end their friendship if they tell. Even the gay sex part isn't a big flag. I mean, if the boy is confused sexually, it is no different than boys at that age checking out their parents magazines in between their mattresses. Even if not confused sexually, it could even be a matter of it freaking him out at that age to picture girls naked so the images of 2 males isn't as threatening. Its a confusing age for children, even more so if the child is bipolar and is facing hyper sexuality issues as many bipolar people face. This doesn't mean that this woman needs to envision her son being some future sexual deviant abuser by all means. And if it is a curiousity related to sexuality confusion, it might help her realize her son is questioning his sexuality and perhaps needs some support with that and understanding that it is okay to explore his thoughts on his sexual orientation.
My older sister, the one I've only just met, has a 20 year old son and a 18 year old daughter. My niece is bipolar. My nephew identifies as transgendered. he is in the beginning stages of a very long process of working towards surgical intervention to become a woman. His path has been a hard one and I know my sister has mentioned some awkward situations in his younger years when most kids start exploring sexuality and understanding sex and wanting to understand more about it. Where boys and girls were starting to kiss etc, her son was unaware of the impact his own similar exploring because his exploring attempts were made towards boys. He is not a abuser, he was simply young and didn't know any better and thankfully my sister learned about it and was able to talk to him and help him figure out what was going on with him. I'm saying all of this to say that this other boys mother need not freak out that a boy was exploring sex or porn, but that he certainly is at the age that she needs to recognize his curiousity and help him learn what is appropriate etc. And the sex/porn thing aside, he could use a good lesson in how to treat his friends so he doesn't lose them. None of this behavior of that boys raises huge flags for me at all. But does raise a good point for this other mom, that it is time to have some good talks with her son as he is maturing and with that he needs guidance in what is appropriate etc.