Is it permissible to pray for the destruction of a country due to the prevalence of sins and tribulations within it, so that it may serve as a lesson and save the righteous?

There is no harm in supplicating against wrongdoers and those who commit sins. Indeed, the prophets, peace be upon them, supplicated against their disbelieving and oppressive peoples. Therefore, your friend should specifically supplicate against the people of disbelief and sin, rather than the believing and righteous, so that what befalls them may serve as a lesson for those who escape.