I'm going to meet my friends before finishing work for my mom
Why would you want to do that?
Because I don't want to make my friends mad at me.
If you started a job for your mom then you should finish it before going to meet your friends. A job started should be finished.
Maybe I should make something up and tell my mom that I can't complete the work tonight.
That so inconsiderate of you. Your mom asked for your help, and you should work until that help is completed. Any decent friend should be able to understand that and give you the time and space you need to help your mom.