According to what you said, I can confirm that your friend has used the student card illegally! There is no problem with the fine of MTRC! However, according to the relevant regulations of Shenzhen Metro, your friend's fine is an administrative fine, and you need to issue an administrative fine. Your friend needs to sign to confirm that the administrative fine has three parts. If your friend pays a fine, some of it will be given to your friend as a voucher!
According to what you said, after the opening of Line 2 (shekou line), 70 yuan can only be fined, because the illegal use of tickets is ten times the highest fare charged in the whole process, and the amount of administrative fines cannot be changed! It is impossible to say that you can accept 60 yuan without 70 yuan!
Personal suggestion: Ask your friend if there is any administrative fine. If there is, wait until Line 2 (shekou line) is opened! If the subway staff doesn't show you any documents and fines you 60 yuan, it must be illegal operation. You can go back to the relevant subway station and ask for an explanation! Don't worry, there is no evidence! Because every subway station in Shenzhen Metro has many cameras!
Finally: if your friend already has an administrative fine and signs it! So, this is business!