It's no secret Dubai has harsh rules, he is an adult, he was responsible for following the law. At the very least as the ones who picked the destination, his parents should have clued up on these laws and made sure he was aware if they thought he was too young/irresponsible to look them up himself.
If you plan on drinking or shagging about with strangers on holiday, the onus is on you to check the legalities like drinking age and age of sexual consent. It would not have been hard at all to find the information that the legal age for sex was 18, and he knew she wasn't 18. Isn't it also illegal to have pre marital sex in Dubai, so even if she was 18 he still would have been breaking the law of the country he was in. Usually overlooked for tourists or not, the law is the law.
He thought with his penis, had casual sex with a virtual stranger, and broke the law.
It is not on the girls mother that he's in prison. She is religious and believes pre marital sex is a sin, no doubt she's distressed her daughter had pre marital sex. It may not be her choice if her daughter follows the "rules" of the religion shes been raised in, but she was not wrong to report a sex crime having taken place.
At the end of the day, he's broken the law of the country he was in, and had sex with someone of an age they class as a minor. He's lucky to have been given the minimum custodial sentence. Yes, this will affect the rest of his life with a sex crime on his record, BUT it was a very simple thing to avoid, he made the conscious decision to have sex with that girl, knowing her age. Ignorance of the law of the country he was in, is not an excuse. He had every oppertunity to check the age of sexual consent just like you would check the legal age for drinking if you plan to drink.
No sympathy for him at all, it was completely avoidable and 100% down to his own decisions of sexual gratification over following the law.