Wow, what an interesting young lady.
From Medicinenet.com:
The overall prognosis for lung cancer is poor when compared with some other cancers. Survival rates for lung cancer are generally lower than those for most cancers, with an overall five-year survival rate for lung cancer of about 16% compared to 65% for colon cancer, 89% for breast cancer, and over 99% for prostate cancer.
That speaks volumes to me. Without treatment I doubt her lung cancer would just go into remission.
Maybe you can do a short internet search and get just enough info to make yourself dangerous. Ask her a few questions, not in a nosey way but in a more inquisitive way -- make up some story about a friend who has/had LC. As the girl if it was small cell or non-small cell or what stage. (or even make up a 3rd type and see if she picks that one) You can ask her if she'd recommend her doctor b/c you have a colleague on the DIS in your area who needs a second opinion.
I don't normally advocate any sort of lying or surreptitiousness but I think you need to bring out the big guns.
A complete remission is when the tumor disappears so that it cannot be seen on X-rays. However, unfortunately even if a lung cancer goes into a complete remission, it usually eventually starts to grow again.
Here is an interesting website about teens and cancer.
http://www.kidshealth.org/teen/diseases_conditions/cancer/types_of_cancer.html
You're in an awkward position -- but then what parent of a teen isn't. Yours just is a bit more unique. If you can gather more information your son might be more willing to see your side of the story.
To me, it seems odd that a parent of a child (even a teen) with cancer wouldn't want to be around. I know it's hard to see a child ill, but cancer? I know she's a teen, but to be so nonchalant about having cancer -- twice! -- doesn't seem 'developmentally appropriate'.
As for the Crohns, she may very well have that. Crohns is an autoimmune disorder of the gastrointestinal tract. The symptoms can be episodic (come and go) w/o explanation. You can ask the girl over for dinner and in doing so ask her if there are certain foods she must avoid due to her Crohns -- many have to avoid dairy or spicy foods.
Good for you for being concerned! Some might see it as nosy but it's one thing to see your son's heart broken, it's another to see him being manipulated.
Best Wishes!