@Maree72 you can absolutely express your displeasure in the insensitive comment made by your friend. My guess is the member who sent you a private message was just giving you the heads up in case it opened a can of worms for you with a friend that may possibly see the post.
People don't even have to sign up to see forum posts, and a while back we had some members who put up photos of themselves after surgery, not knowing it can be seen by all, whether a member or not on the forum.
Your main concern is how to respond to people who make such comments, and I am still trying to figure that out myself, as sometimes even now nearly 6 years on I still sometimes get taken aback by some people who should actually know better. The latest one was a good friend who finally said to me last year that they have a cure for cancer and are keeping it from people, and sorry to inform me! Further saying I am naive to believe otherwise.
I once had an acquaintance/friend embarrass me at a birthday get together in a hobby group I was attending not long after my diagnosis. She said in front of everyone that I shouldn't have any cake as the sugar will feed the cancer. If I had thought quick enough, I would have said, don't worry about me I already have cancer, it is you that shouldn't eat the cake if you don't want to get cancer!
Seriously though, some people just say really crappy stuff to people with cancer at times. Maybe just say something like, I FIND THAT TO BE A CRAPPY COMMENT!
It can be very disappointing to hear upsetting comments like you have experienced. I hope you can put those people who make insensitive comments back in their place, or just walk away, and/or ignore. Wishing you all the best with your treatment. xx