American mother to British child visa query

My ex-wife is American and my daughter is 9 years old and dual nationality, American and British.
My daughter's mother came to the UK with me in December 2004 and moved back to the States with our daughter (then just over 3 years old) in June 2013 after our divorce.
Our daughter is now struggling to cope with the separation between her and I and her mother wants to move them both back to the UK.
Obviously there is no problem with my daughter coming into the UK as a citizen, but what are the options for her mother?
Any help or guidance is greatly appreciated.