一个定语从句的主谓一致问题I,who is your friend,will do my best to help you这里的is用的对么?

对的,因为主谓一致,用作主语的名词词组中心词和谓语动词在单、复数形式上的一致,就是语法一致.也就是说,如果名词中心词是单数,动词用单数形式；如果名词中心词是复数,动词用复数形式.例如：
This table is a genuine antique.
Both parties have their own advantages.
Her job has something to do with computers.
She wants to go home.
They are divorcing each other.
Mary was watching herself in the mirror.
The bird built a nest.
Susan comes home every week-end.