As per your query,it looks like lipoma
.It can turn into cancer called liposarcoma
Please do not waste time.
Meet your surgeon and get it diagnosed.
You can go for a simple test called Fine needle aspiration cytology(FNAC) which tells whether the swelling is benign or cancerous.Ultrasound
of that particular area will also gives some clue.
As it is freely moving,it cannot be sarcoma.
But confirm the diagnosis soon.
wish her good health