Nexus 5 turns suddenly off. Could it be a contacts problem?

Hello everybody!

My brother sent to repair his Nexus 5 for broken screen. Since then the phone sometimes suddenly turn off and it seems like the battery detaches from it's place because it is not a problem of charging it.

What's the problem in your opinion?

I thought about a contact problem, is it possible? Or do you think that the battery was broken? I've seen that, during the teardown, the boy had to force a lot the battery to take it off.

Thank you in advance!

Matteo