Is back for more an idiom ?
" She keeps going back for more".
She continually returns for extra doses of the previous thing she was receiving....
For example, if a person is subject to abuse, but cannot extricate herself from the situation, inspite of attempts to distance herself from the abuse, she can be said to be 'going back for more' (of he same abusive treatment).
If you are talking about an unhealthy relationship between two people and
" She keeps going back for more", Yes, that is an idiom to mean one person returns to a relationship, only to continue experiencing the same behavior in a dysfunctional relationship.
However, it can also mean to return for more. She liked the cake and she keeps going back for more.