Question about period after giving birth

I have a friend that just gave birth to twins may 22nd. She bled for about a week and half. I know you are more fertile after birth but she was asking me when should she start again. Would it be around the 22nd? She's on a normal 28 day cycle. I can't remember with mine. And she likes to keep track of that but doesn't know where to start. Any help would be greatly appreciated. Thanks