Honestly, the idea of borrowing a phone (and I know it‘s not necessarily yours) is somewhat preposterous.
Letting a complete stranger install an app on my phone, from a bank I don‘t know, anf from country I‘ve never been to. Availability on Play Store or not - that‘s just extremely bad from a security standpoint. You know, he or she might be doing online / app banking on the device itself.
Just ask yourself: would you lend a stranger your mobile phone, to install a banking app from, say, Ukraine, Indonesia, Ghana or Brazil? Does that sound a good idea?
Also, buying one might not be that good (or even possible) an idea if you got robbed or your hotel room burglarized while sitting at the pool. Cause they will often take your wallet, cards and phone. Even if you do manage to receive emergency funds, you might to have better spend them on identity documents and transport (embassy, hospital, back home).