Overtime 'under the table'

this question is actually in regards to a problem a friend of mine is having. it should be noted she is not in any supervisor role, and is eligible for overtime.

she is contracted for 40 hours a week, but regularly works over this - sometimes up to 60 hours. she is not paid overtime for this, instead her employer gets around this by making her fill out two clock in cards, and the second one is paid 'under the table'.

this system was basically forced on her when starting work, and she didn't feel in any position to speak against it. is there any point in her attempting to speak up about it, or will this get her into more trouble than it's worth? is it even worth anything, or would she not be due any money, due to having accepted under the table money since starting work. she's worked there nearly 3 years.