Hi Olivier,
The question that Matt is answering is quite specific - is a "coming soon" page bad for new domains as Google seems to prioritize new domains. The answer is "no" for this situation.
The situation of your friend is quite different - he has an existing site which is generating some traffic. Unless your "coming soon page" is extremely interesting & rich in content, it will not rank for any keyword - basically you're reducing your site to a one page site, so bounce rate will inevitably be very high.
If you want to proceed, you just have to put up a redirect rule that is redirecting all url's to the index page of your site (which would then be the "coming soon" page).
rgds
Dirk