There's no love in the relationship from the first place anyway so just stay friends & enjoy life! However if you want her company your best option is to be friends cause she's doubtful with her feelings & you can also help them to reunite instead of fishing in the middle.
So a girl broke up with her ex. Asked to start a relationship 1 week later, 2 days after asking (she wanted to wait a week) started dating ex again, wants to be friends now.. What do I do?
Do? There is nothing to do .. Except move on. She is unavailable .. Period.
To be honest with you, she's a confused woman! Yes a woman! She really can't decide! Go to her and tell her that this is not good for her, she should have the right choice, and make the right decisions!
She's doesn't love you , nor really like you, she's fond of her ex and she can't forget him! Just give her the advice I gave you so that she has a better life.
( l answered this keeping myself in place of her):)