There is a guy he is from malaysia, he has got a student visa which runs out this month but he's not a student he asks someone to sign in at the college for him. He is working and he is planning to get an extension on his visa and is planning to marry my sister to stay in country.What can I do?

Not enough information. What country? Is he marrying your sister only for the immigration benefit and not to have a shared life together?

It's West Yorkshire, UK the OP is from.

What do you mean what can you do? I would not involve yourself, he is on a student but not studying so he is in the UK illegally. If he truly wants to go to school now he has to apply for a new student visa.

If he wants to look into applying for permanent residence (or the UK equivalent of this) he and your sister need to find out the application procedures. I would definitely say that signing up for school in your name for another person would be illegal however. Depending on immigration alws in the UK he may have to leave the country until his student visa (or spousal visa) is approved.