"I will let her go."
Translation:Tôi sẽ để cô ấy đi.
I don't know if it is accepted (you must try it to know), but it is a correct translation.
"Cho" means allow. The sentence with it will mean she is arrested (or in some occasion where she should not be allowed to go), then you decided to let her go.
"Để" is much like the English "let". The sentence can have the meaning above, or just you don't persuade her to stay anymore.