Bear in mind, 18 years is the minimum sentence. After that time he will face a parole board and he would only be released (probably on license) if he was thought to have demonstrated that he was ready to be rehabilitated, had sincerely repented, etc. He may well end up dying in prison.
What I found most striking is that he is the son of a high-ranking Nigerian family, and I am assuming by her given name that his victim was also Nigerian-born. I believe he was a student, so if he was released after 18 years he would probably be deported back to Nigeria. At that point his life would basically be over due to bringing shame on his family. So please don't think he will have a cushy time. I know that is scant comfort to her family