16 years old got engaged to her 31 yrs old former elementary school teacher

Ok this is 100% TRUE story and I couldn't believe it! My youngest sister has a friend, who at age 14 started dating her elementary school teacher. It was a big deal and they kept it as a secret as he could have lost his job. Oh well, she lost her virginity to him when she turned 15. Her super strict parents, who didn't even let her go out with my sister and her friends after 6pm were happy, that their little daughter dates a smart older guy!!! Btw he was dating another of his pupils before her! But her parents don't know that. Now, 2 days ago they got engaged?! She's 16, he's 31 and it makes me feel sick! I know it's not my business especially when her parents agree with that, and sure he's not her teacher anymore, but I think it's absolutely disgusting.