Can I back up someone else s iPhone on my iPhone?

Can I back up someone else s iPhone on my iPhone? - 1

Yes, nandroid backup.

Yes there's an app ahh I forgot what it is called you just type his ID of phone and done.

As I know, you can't backup someone else's iPhone on your iPhone, because they are in different Apple ID. Only Android phones can do that.
If you just want to transfer files between two iPhones, this Mobiletrans can help you do the job. You just need to connect your two phones to computer with the USB cables, select the files you want to transfer, and click "transfer" button, that's all.
http://www.reviewstown.com/phone-to-phone-transfer.html