What is the relationship between a teacher and spiritual friend?

The disciple of the Buddha needs to understand The Four Noble Truths. The Fourth Noble Truth is The Noble Eightfold Path. The Four Noble Truths and The Noble Eightfold Path are central and fundamental teachings in Dhamma that are to be studied, understood and practiced in order to stay on the Path that leads to all the Noble Fruits:

sotāpanna (streamwinner),
sakadāgāmi (once returner),
anāgāmi (non-returner), and
arahat (the liberated one).

Mastery of The Four Noble Truths and The Noble Eightfold Path are sufficient for liberation from suffering and the realization of Nibbāna.

The Buddha said that when his physical body is gone then the Dhamma will be your teacher. The Buddha did not assign any person to be the teacher or to lead the Saṅgha. The Buddha also taught that spiritual friendships (kalyāṇamitta) are important to help reinforce our commitment and understanding of the Dhamma. Kalyāṇamitta means a good friend, i.e., a spiritual friend who gives advice, guidance, and encouragement. Do not be mistaken that a spiritual friend is your teacher. The teacher is the Dhamma and spiritual friends are companions that support each other in understanding and practicing the Dhamma.

Spiritual friends in most cases still have defilements (ignorance, greed and ill will) that distorts their understanding and practice of the Dhamma. Therefore, the behavior (speech and actions) of spiritual friends must be investigated to determine if they are in line with your understanding of the Dhamma. Sometimes the behavior of spiritual friends will be in line with the Dhamma and at other times they will not reflect the Dhamma. Cherish the behavior of spiritual friends that are in line with the Dhamma and ignore the behavior which is not true to the Dhamma.

The study and reflection on The Four Noble Truths and The Noble Eightfold Path will determine what behavior is in line with the Dhamma and the behavior which is not in line with the Dhamma. If after study and reflection of the Dhamma you are still confused about your own behavior or that of a spiritual friend then consult with numerous spiritual friends, compare what each has to say, and do your best to determine if the behavior in question is in line with the Dhamma or not.