I don’t understand how this person has stage 4 cancer.

https://shine365.marshfieldclinic.org/cancer-care/a-guide-cancer-stages-terms-and-side-effects/

A friend of mine has stage 4 cancer and goes swing dancing every week

Some people can be deathly ill and look and do fine

She has cancer, I know who you are talking about. Everyone is different and this girl is certainly going through a lot. She tries to enjoy life when she can.

I knew someone wonderful who ended up dying of cancer - she looked incredible when I last saw her before she passed only a few months later.

Make up, fake tan, a smile and a good wig can cover a lot of pain and exhaustion.

No disease or disorder affects everybody in the same way. Some folks with cancer are appearing to be 'just fine' (even if they aren't) and some look sickly all the time. It's really weird for you to assume this person isn't sick based on your narrow view of illness. Is she supposed to just lie miserable in bed all day to satisfy your image of what her illness 'should' look like? You're probably one of those people that tries to 'prove' disabled people aren't actually disabled because they don't act/look the stereotypical way you imagined. Disrespectfully, fuck off with that shit.