Possible Fraud

We have physical custody of my 10 year old step son who has a physical disibility and lives full time with my husband and I. His biological mother does not pay any kind of support for the child and lives in another state and he see's her maybe once every other month. We recently found out that she is drawing a Social Security Disability check on him from the state that she lives in and has for quite some time. How can she legally collect money on a child who has never lived in that state and what can be done about it? Also now that we know about it can we be held legally responsible if we don't report it? (We are afraid she will take it out on my step son if she finds out we told.)