Are you serious? this person has been stealing off me for 15 years! and you, if you pay tax. of course it's serious. I bloody hope they take it seriously. As someone said here the other day - if someone came in your house and stole thousands of pounds off of your side board, you would want them punished, and there is no difference here.
Many people get put in prison for fraud, i guess its more likely the bigger the amount, and 15 year worth seems quite big. also meant to add, that if you get convicted it can prevent you having access to benefits in future (even if you pay it all back)