iPhone iCloud activation lock with dead person

So basically I fix iPhones and customise them, my grandad had a friend that recently passed away who said that he could have his computer and gadgets as he knew that he was going to pass away with cancer. This was a common interest between the two of them and because my grandad had just upgraded to the X so had no use for it so he gave it to me knowing that I customize them, I customized it into a custom iPhone housing with a glowing logo Etc. But then I went to set it up and realised that it had an activation lock, I contacted Apple and they said I needed the original death certificate and the iCloud email address but my grandad does not have that as he used his gmail as his normal email so he doesn’t have his iCloud and this man had no partner or living relatives. I’m a 14 year year old boy who is hoping to sell the finished phone on eBay to buy my mum a birthday present and also buy her a train ticket to see her mum who’s in hospital. Please help as this is very important to me. Any answers are greatly appreciated.