I don't think the world "third world" is offensive, particularly (but then I have better things to do than go around being offended by anything and everything). I DO think that it's a meaningless word that suggests that the person using it does not actually know very much about the issues or country that they are trying to describe.
"Third world" has not been used as a serious term for decades, and is devoid of any proper meaning or definition. The commonest terms used these days are "low income" "middle income" and so on.
But just substituting "third world" for "low income" here, would be to completely misunderstand Tunisia. Tunisia is invariably defined as a middle income country --in fact, it's generally defined as upper or high middle income, or using terms like "emerging." It's not wealthy like the UK, but it's not in the same category as Nepal or Sierra Leone etc. and it is actually one of the more functional places in North Africa.
en.wikipedia.org/wiki/Economy_of_Tunisia
None of this alters the fact that many young men in Tunisia will be desperate to come to the UK. A major issue of most of the North African and Middle Eastern countries is that they often have excruciatingly high rates of unemployment among young men, due to high birthrates (=lots of bored young men) and stagnant economies. So you get young guys hanging around, precariously employed here and there, often not having a lot to do, and hoping to try their luck in the UK instead. It's understandable (but it's still a scam).
As for the situation--I think it would be nice for holiday reps etc. to point this kind of thing out. But they are not legally obliged to, and yes, the women who fall for this kind of stuff are very very stupid.
I am LOLing at the poster who made some comment along the lines of "Would you be saying the same thing if she were an older guy with some young girl?" Err, yes, in my experience, people are even quicker to say how disgusting that kind of thing is. And yes, I would be concerned about both parties in that kind of situation. It's risky for the young woman concerned, but young women are also capable of scamming deluded older men. I live in East Asia and have seen this kind of thing for myself.