domain mapping and https subsite

Hello,

I have found some topics about domain mapping and https but not with my question.

I would like to know if my website is entirely SSL encrypted with a https://www.mysite.com and then domain mapped and if a Customer buy his domain like .www.hissite.com pointing to .www.mysite.com/hissite

What will happen?
https://www.hisite.com will work?

Or as the SSL certificate is for mysite, he will have a warning message?

Thank you

Ludovic