In the UK, you will definitely here people using 'plenty' with both count and mass nouns, though some prefer it to be used only with mass nouns. We can use a lot of/lots of/ loads of with both count and mass nouns.
You should have asked your classmate to lend you a pen:
Could you lend me a pen?
Could I borrow a pen?
In some regions, people say 'borrow me a pen', but this would be considered incorrect in standard English. The 'it would be helpful of you if you could lend me a pen' structure doesn't sound natural to me, just use could/would to show politeness and say please- asking for a pen isn't a major favour.