My mom has a variety of serious illnesses going on. Some are physical, and some are mental. I see her on a regular basis and do some emotional triage in the time I spend with her, in addition to other things. During our time together, we have very personal and serious discussions, some of which revolve around quality of life and the right to die in dignity.
I don't tell my partner everything. Why? Because those talks with my mom were privileged. She trusts me that she can say anything and it will go no further than our safe space. But, sometimes I do share aspects with a long time friend who also shares with my mom. We have a confidence that outdates our husbands and partners.
When I don't share discussions with my partner, I'm not excluding him. I think this is the sore point for you: you feel like you've been left out of the picture.
I would talk to your partner, and be there for him. Put your feelings to the side for now, and focus on him. He just found out his mom has cancer, and there is a lot on his plate. Open that door of communication, and let him know he can trust you to have those sorts of talks. Perhaps when he starts to share what is going on, the conversation will naturally lead to why he first confided to his friend. But, maybe it won't. That is something you will have to find out for yourself, and an answer you won't find here.