Someone took a loan from a Western bank and then defaulted on repayments due to the annual interest. How can he repent? And what is the ruling on the property of non-Muslims?

Regarding the first question, even though he cannot pay back such debts, they remain payables to be fulfilled by him. He must ask Allah for forgiveness, regret his deed, and never do it again.

As for the second question, it is impermissible to transgress against property of non-Muslims by cheating, deception, stealth, or whatever. Your entry visa to a certain territory is like a “covenant of security”, which is incompatible with taking others’ property illegally. If you obtain something from a non-Muslim as a free gift or by purchase or some form of transaction, there is no impediment to this. But to appropriate others’ property or swindle them because they are non-Muslims is impermissible in Shari`ah.