How do I tell her

So I just found out that my best friend got back together with her ex. I don't like the idea of it at all, they literally sneak around. And they last time they where together he spent her money that she earned to gamble and go to the pub and just pay for everything. I know that she didn't like at as she told me like she even lied about losing her card so he wouldn't use it. I also find it weird that she went back to him, as when they where together she did show a big amount of interest in me, like she kissed me and everything. And once they broke up she kept saying that she wanted to be with me, I told her no as she had just gotten out of a long term relationship and needed time to really figure out what she wants.

I don't know how to tell her that I think that this is a mistake, she deserves better than someone who mooches off her, and deserves someone who will actually go out in public with her and not hide the relationship.

Like I don't want to be that friend to tell you what to do like I don't like your boyfriend and don't think you should be with him again.

Ugh I just don't know what to do