Removing BIOS' password in XPS 15 l502x

Hi everyone. I have an issue with passing BIOS. My brother now is in prison, but his laptop is left at home. I understood by removing the HDD that he deleted everything from his laptop (even OS) and put a password to BIOS. My grandmother asked him about the password, but he forgot it. At first, i tried to contact with dell's support, but they asked me for retailer's  name. We bought this laptop in Dushanbe and even my brother don't know the retailer's name. At second, i searched the whole internet to find the unlock key, but i didn't find it. At third, i removed for 24h CMOS battery, it also didn't help. What can I do in this case?