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There's no true right or wrong or universally correct answer in this situation because it really comes down to those two indidivuals--the LW and his gf.
When I was diagnosed with cancer nearly 12 years ago, my gf at the time and I were semi-broken up (this would be a pattern we experienced time and again in part due to my relationship immaturity and in part to issues related to her bipolar disorder). When she got the news, she rushed to my side and really was there for me in ways I needed most. In ways my family couldn't be because of the emotions they were dealing with. That said, there came a time when she couldn't deal with my being a survivor and the quirks and such that comes with that territory (anxiety around anniversaries, etc.). And she couldn't be there for me in other ways I needed down the road as I grew emotionally stronger. That's life. Some may have been about my cancer, most of it was not.
But I will always be grateful to her for those weeks and months in which she was there for me. That relationship ended nearly a decade ago. My loving partner of 6+ years is emotionally capable of being there for me around scan time, giving me comfort and support or a swift kick in the butt, whichever I need most. We communicate very well, very openly about our fears, hopes, dreams. I'm so grateful for her, but also for the good that came of that earlier relationship, for it helped me to grow into the person I am today.
Just food for thought. I wish comfort, peace and understanding to the LW and his girlfriend, and hope she can celebrate many, many years as a cancer survivor.