What are relationship red flags

Hey y'all my friend is in a really weird relationship. She's getting married at the end of this month and all the older ladies and aunt figures in her life tell her not to worry and that her relationship will get better after the steessof the wedding goes away. However I feel that he does not respect her feelings and is not forgiving just from what I've heard. She told me that he made her promise not to share information about their relationship with anyone else. Which is kinda weird.....

What are some red flags that I should look out for? And do you think I should say anything else before she gets married. I've already told her that I would not marry him if I was her. I'm a bit worried about her but I know that I dont know the whole story. Any advice would help.