It is a very positive sign that your daughter came forward and was able to tell you about what had been happening.
The ages make it not legally abuse unless the age of criminal responsibility in the state it happened in is unusually low....
That is not to deny that real emotional damage can occur.
Was the matter reported to local child welfare people or did you tell the other girls parents?
I would hope that it was and that the other child and family is being seen by someone skilled in working with children with problematic sexual behaviour. The causes of problematic sexual behaviour can be complex and difficult and a seven year old does not act with the same comprehension about what she is doing as an adult.
Your daughter is seeing someone. Are you also receiving skilled support? A major factor in your daughter's progress will be how able the important adults around her are to support her calmly and empathically without being overwhelmed.
You have also been clearly very distressed and I think it would be great for you to have your own space to get support for your own feelings.