Saying that nobody should ever do it isn't realistic, sometimes things go wrong on a Friday, sometimes you can't afford to wait 3 days to watch them become even worse.
It's good to instill in your team that Friday deployments should be done only in special circumstances, or they become a habit (and people inevitably end up working Saturday). But yes, sometimes it's necessary.
Don't deploy when you won't be around to support it is a better description; but it doesn't roll off the tongue as easily.
Really, it will come down to a cost/benefit analysis. Are your chances of the site going down due to being hacked over the weekend higher than the chances of a last minute update taking down the site?
The answer is almost always no (much to a sysadmin's chagrin). If the answer is yes (i.e. another heartbleed), then you are probably going to be working through the weekend anyways.
"Only deploy on Friday if you like working weekends"