My father-in-law had forgot his Apple ID on his iPhone. He had his friend enter his details to download some apps on to his phone. He's since reset his password and set his phone back to his own account (both the iTunes and iCloud are using his email address). The issue is, when he goes to upgrade some apps it's asking for the password for his friend's email account? How can we resolve this?
Chances are the friend downloaded those apps under his/her Apple ID. Thats why it's prompting that Apple ID username/password.
Simply delete those apps and reinstall them under your Dads account and everything will be fine.
If these are Apps that your friend has paid for but your dad has not and thats why the friend downloaded them to his device so your dad wouldn't have to pay for them, then thats a different issue. look into family sharing and see whats going on there from both parties.