They wouldn't be mersyndol would they? If not, they're virtually the same thing (a couple of mg's different in some aspects).
From what I understand, it's pretty safe - in fact in the past I've taken two mersyndol before going to bed after a weekend has "whizzed by"...
The sedatives he's got are over the counter, so there's nothing
too powerful. He shouldn't have more than 3 at the most though, because paracentomol is harsh on the liver, and his liver will already be a bit punished. Because the dose of codine is realtively low, it'll be the doxylamine succinate that has the noticable sedative effect - it might be worth getting some sleeping tablets with only that in them (restavit has 25mg of doxylamine succinate). I started a thread on that substance a little while back - I'll dig it up for you...
I'd also just like to mention that when going out and taking large doses of amphetamines (pills are amphetamines too remember) it's natural not to be able to sleep. The best cure is time - don't try to go to sleep when you can still feel the drugs, it's better to wait 12 hours or more if at all possible.
{edit}
Doxylamine Succinate - there's some really good information in that thread that relates to your question too...

[ 30 March 2002: Message edited by: Pleonastic ]