And it could equally be the water/chemicals in old royal residences (hence the double cancer diagnosis amongst its inhabitants), his family genetics, the stress of his own childhood or adulthood (including his betrothed young wife having violent outbursts and dying tragically, and him and his lover being vilified figures in the UK and worldwide for decades)... Honestly a spoilt runaway child is but 1 element that may or may not have factored in at all.
I also genuinely don't think Harry (and his statements) mean enough to Kate to have made her stress day and night over it to the point that she got cancer! The media (Waity Katey, nude pics, family defaming, etc) has definitely done worse to Kate than Harry over the years.
Not a Harry supporter by any means but it's just brainless and vicious to say Harry caused anyone cancer.