At this point your mum actually isn't in the wrong or committing fraud (currently)... She signed a tenancy agreement years ago. It has now lapsed. So she has no legal connection to the property. It would be hard to prove the previous wrong doing.
If her name is still on a tenancy agreement its because your uncle forged her signature. What she needs to do, is nothing and claim ignorance. If there is any fallout in the future, due to her name being still on the agreement, the response needs to be "Not possible I moved out 6 years ago. I guess that other guy didn't tell you and or has been committing fraud by forging my signature on new agreements... thats a you/him problem"
It would be easy for your mom to prove she doesn't live there currently and given the tenancy is 7 years old it would difficult for anyone to prove she didn't move in....
Do nothing, ignorance is bliss - it will blow up in your uncles face at some point.... If you want that to happen sooner... just track down the owner and mention the illegal sublet (anonymous if need be). If they try and implicate your mother, again ignorance is bliss... stick to the story "i moved out 6 years ago, no I haven't signed any tenancy agreements since...sorry". Don't want to be implicated in that lie..."I have no knowledge of any of this, I guess my (proven) dodgy brother forged my signature from the start"
It would be very hard for the landlord to prove your mother did a fraud at this point if played smart...Uncle well he can be shown to be forging signatures and subletting... the axe will fall on him... I wouldn't panic.... The police are not going to put your mum in Prison for bending the truth on a 7 year old lapsed tenancy agreement