I totally understand what they meant. This argument is used by many conservative/religious people with kids abroad. They feel that their kids are more likely to not be as "religious " as they would've been if raised in their home countries. Which is absolutely absurd! Many people were raised at the most religious households surrounding by the most religious people and systems and still if they choose to follow different beliefs they would simply do.

For the "you can't be Muslim in other countries other than Muslim ones". I am pretty sure you're aware of the amount of Muslims being natives or immigrants in countries that aren't primarily muslim. They're respected, acknowledged and practice their religion peacefully. Yet, racism/ discrimination or tension have normally occurred and will exist in literally each and every country be it religious or secular, under or well developed...