Can a mother of a 1 year old child who is a british citizen by decent achieve residency in UK?

The Child was born outside the UK in 2009 with a woman considered a common law wife. The father is a british citizen otherwise by decent (as he was born in England to a british parents in 1971). As the father needs to move back to the UK, he would like his child and the child's mother to live in the UK. For the child there is no problem. However the mother is a Lebanese citizen.