Since he's deleting the messages, I don't think you need to say anything unless he is answering her. As long as he's not, she'll get the hint after a while and stop. It shows he thinks more of you by deleting that stuff. You'll be fine. Just make sure he doesn't answer or she'll keep doing it.
My bfs ex wont leave him alone, she texts him Reminiscing about their relationship and it really annoys me, he deletes the messages and ignores her. Should i ask him to tell her to stop?
If he's already ignoring her then you don't need to worry about it. I think you should tell him to block her.
He could block her number.
Your best bet is stay out of it for now and let your BF handle it or else youll only open another can of worms and prolong this and she will take it out on him. Unless she turns dangerous, ignore her as she will get tired of her little game and let it go so be patient but from experience , keep an eye on your BF so that he doesn't falter here. Good luck