Hi, and thanks for your question. We'd advise your friend to contact the Land Transport Authority (LTA) directly to inquire about this. On the other hand, your friend could head down to the inspection centre and perform the inspection after informing the staff that he/she missed the inspection date.
We'd suggest your friend does this ASAP as missing the inspection can be seen as an offence, even if it wasn't missed on purpose.
You can contact LTA's customer service hotline at 1800 2255 582.