My Nephew Has a Difficult Time, Severing the Affair with His Married Instructor

A Q&A, translated…
Q: They’d Been Involved for Over a Decade Now, How Can I Convey to Him, that Letting Go is Best?
The capable worker in the publicity, Jia is troubled lately, she and her husband didn’t have children, they’d loved her sister’s son as if he were their own, plus there was enough space in their home, as the eldest nephew got into a high school up north, naturally, he’d, come to stay with her.

like this???
Started in the second semester of his first year in high school, he’d turn home late a couple of day out of the week, and called her on he was staying after school to study. Naturally, Jia and her husband trusted him, until one evening about six months later, as Jia’s husband was going on a business call to drink, he saw their nephew walking very close, with a woman of about thirty interacting intimately. He’d thought that this was a bit odd, rushed home immediately, and told his wife about it, “Could this be a student-teacher love affair? Your sister sent her son here to us to look after, what if something were to happen?”
That night, as the nephew came in at around close to eleven, he’d still told them that he was studying with his classmates, Jia couldn’t hold it in any longer, confronted him, “Are you dating that female instructor?”, the nephew explained, that they’d just had a lot to talk about. That weekend, Jia’s older sister rushed to Taipei, tried to get through to her son, and asked someone she knew to investigate—that instructor is twenty-nine years-old, still single.
And the sisters discussed the matter, and believed, that as the boy goes off to college and start to date, then, the two of them would split up naturally, there’s no need to make a scene of it right now. As the nephew entered into university, he did, start dating, and, the female instructor got married shortly after too, they’d stopped seeing one another for a while. But, they’d, reconnected again; the nephew would spend a day with his girlfriend, a day with the instructor. As he’d started working, he’d not severed off the ties with the teacher, instead, he’d, broken up with his long-term girlfriend. Jia’s older sister kept begging her son not to break up someone else’s marriage, and the son got tired of hearing it, and just, moved out.
Seeing how her sister became more and more depressed over the matter, Jia wanted to know, how she can help her older sister out?
A My Advice
They’d been together since his first year of high school, that’s over a decade, and naturally, it’d shown, how well they got along together, but, they must’ve talked about how they couldn’t be together (after all, he couldn’t be like the French president, Macron.); but since the child is already adult, just let him be. If he gets sued for adultery, then, it’s his responsibility, although it’s adultery but it’s, a minor crime, he must shoulder the responsibilities on his own.

passing notes here, not my cartoon…
This, is truly serious, when your child or someone else’s child that you know starts a love affair with someone who’s married, it usually doesn’t end well, and, no matter what you (people on the outside) do, you can’t deter the child from getting infatuated, and all you can do is to wait for the love to faze out on its own, and if it still won’t, then, there’s still, NOTHING you can do!