What was which one thinking?
I think Carrie probably has high political ambition herself, and five years in Downing Street as prime minister's consort seemed better than a lowly internship.
At least, I can't imagine she's stayed with Johnson out of true love or the belief it will be a lasting relationship. My guess is she's sticking it out for the wedding because the divorce will serve her better than being just one of the discards he didn't even marry.
I don't usually speculate on "slebs" marital shenanigans, but this one may affect the country's political life so may become our business whether we want it to or not.