Typically direct family member or employers. I wouldn't mention that the employer is a friend. How long have you been contracted to the business? It depends on the embassy you're using. I'd seek clarification from them first.
Once your reason to stay in Thailand ends, (work) so does your permission to stay. You also need a cancellation letter from your employer to get the work permit cancelled at DOL, and then take it to immigration to cancel your visa. You can do this in advance.