I do think the girl's mother was out of order for reporting this once she arrived back home, but regardless of your age, sexual orientation, etc, the law is the law.
I know that what I'm about to write is a different situation, but it goes to prove the point that you should always ensure you're aware of the laws in force in the country you're in.
At the weekend, I was out for a meal with family (in England) and had half a pint of cider with my meal, knowing that I would be driving us home afterwards. Now, this was the first alcoholic drink I've had in a couple of years, I knew it would show up if breathalysed (but at a very low level due to the food and time the alcohol takes to metabolise), and that it would be decreasing in potency as time went on, so the longer we stayed after our meals, the better.
My son-in-law commented that if we been in Scotland, there is absolutely no drink tolerance when driving, and my drink would be punishable. I wasn't aware of this law, but, had we been in Scotland, not knowing the law is no defence.
Yes, I'm considerably older than 18, so I 'know' that regardless of what country I'm in, I should make myself aware of that country's laws, as should the parents of the two young people involved in this 'underage sex scandal', and they should have advised their kids of the laws of the country they were going to be in, prior to going, or at least, as early as possible upon arrival, in order to ensure they're at least aware, and can take steps to ensure they're compliant.
These were, after all, 17 and 18yr olds, and not 11yr olds, and as such, should be held accountable for their actions. Just because a 16yr old lad having sex with a 15yr old girl would be unlikely to be prosecuted here in the UK, this doesn't mean that other countries are as lax with their laws as we are.