Strange ip in 2 accounts

My cousin is freaking out. He accessed his sister's e-mail secretly and then later discovered that his ip showed up in the Yahoo activity log. He then says he went to his own e-mail one day and discovered a foreign ip in his own ip activity log. He changed his password every single month like clockwork and never accessed his e-mail in public anywhere to increase security. He then decided to go back into his sister's e-mail to see what her ip address is in the login activity section and noticed nothing unusual except her ip, his own ip, and the very same strange ip he had seen in his own account. The foreign ip seemed to occur a minute after he accessed the account. I told him he is being paranoid. But...he thinks that the police accessed the accounts or Yahoo did if his sister filed an abuse claim. In both cases, his sister's account and his own, the foreign ip is only listed once--yet the same ip in both accounts. He never went back again and deleted his account. Was he being paranoid and deleted his account for nothing or did the police or Yahoo access the two accounts?