With no iTunes or iCloud, how will you back up your phone? You don't want any apps from the App Store? There's a lot of good ones there. I'd hate to rely totally on Cydia.
I would think if you use someone else's iTunes credentials to do this, then your phone becomes part of the account, just as the friends devices. But it could be removed from the account by the owner. Not real sure what you're doing. Good luck with it.
I see you worry for jailbreak iPhone. But nowadays, and specially if iPhone is jailbreak, iTunes is not mandatory to do a backup of the device, a lot of reliable softwares outside there can easily replace all together iTunes function. iMazing software for example is what I use to backup my jailbreak iphone from time to time. iMazing also fully compatible with non-jailbreak iPhone btw.
Jailbreak doesn't mean that iPhone can not access the apps store, instead, not only it can access the apps store, but also can access Cydia store and many other store within Cydia.
Sure he will need his friend credentials to turn off whatever are activated on the iPhone, and then reactivate those using his own credentials.