Should she change her sons name?

My friend had a son before she met her boyfriend. He son was 1 when they met. They are getting married and he is going to adopt her son. Which I'd great he is a wonderful father. Anyway her sons name is Maurice but we have been calling him Mo since he was born( he is 5 now) .She wants to change her sons last name to match his (new) fathers the only problem is that his last name wil be Lester. So his name will be Mo Lester... We plan to call him Maurice more but he gets mad and likes the name Mo better. She does not want to keep his old last name either. What should happen ? Should we keep calling him Mo or what