Replaced screen, but I can't classify this problem.

Hi! So I fixed an iPhone 6 screen (digitizer and all) for a friend of mine because she dropped her phone and shattered her screen. She said the phone was working after the screen was shattered, but it had lost it's touch capabilities so she couldn't use it. I replaced the screen and all with an iCracked replacement kit. I put the phone back together, and when I powered it on nothing happened. It was displaying nothing. Not even a black screen. I assumed that the battery might've been dead, so I connected it to the charger and guess what? It started to make the charging "boing" noise every 5-6 seconds. I connected it to iTunes, and surprisingly it was recognized. Therefore the logic board is fine, right? I took it to another guy who does it professionally, and he said that he didn't know what the problem was. I found a youtube video of a guy with a similar problem and he managed to fix it, but the problem is he doesn't show how in the video and he lives in the UK so I can't afford to pay for all that - I'd rather just buy her a new phone if it comes down to it. Has anyone else encountered a problem like this? And if so are there any ways to fix it?