Marriage regn confusion

In 2008, my friend got married under Chritian Marriage Act. She was a Christian, her husband was a Hindu ... so Hindu Marriage Act Regn was not possible. 

Now, she wanted to change her Sur Name to her husband's instead of father's. They're saying the marriage should be registered under Special Marriage Act for that! Why? 

So, what options does she have now? Should she re-register the marriage under special marriage act, or will be a fresh registration??

Thanks a lot!