Battery won't charge - Lightning connector or Logicboard broken?

Dear Fixers!

My friend owns an iPhone 6s Plus. A few weeks ago, his battery wouldn't charge anymore, so I ordered a new one from iFixit, and everything worked fine again.

Now, the same symptoms occur again: The iPhone gets stuck in a Reboot-Loop, it won't get recognized by iTunes when put into DFU-Mode and the one time I succeeded turning it on it wouldn't charge to more than 1% (so basically nothing).

Now is the point that I don't think that I had bad luck with both the batteries (the Apple one and die iFixit one), so I suspect either the Lightning connector being broken in some way so that it "destroys" the battery over time or the Logicboard having a comparable issue.

No the question: Do you think it might be the connector? Do you think it's worth a try to change the Lightning connector AND a new battery? How high is the probability that the Logicboard is broken and the phone therefore "unfixable"?

Thanks guys!

Alex