its very unlikely that he will die from the amount of oxycontin he is using. snorting an 80mg pill and making it last for 2-3 days is a very small amount. Esp. being that he takes subs everyday when he is not on the oxy that means he already has a tolerance to opiates. So the chances of him ODing on that amount is HIGHLY unlikely.
Very hard in general to OD on Rx. pills because the user knows the quantity and quality he is getting each time. However, when somebody does die of an overdose it is mostly because they mix different drugs together (i.e opiates and benzos or benzos and sleeping pills etc).
I OD'ed a few times when taking heroin and i was a strict opiate abuser(no other drug took my fancy, tried them all) so i know a good deal about oxys and such....Be safe and i hope your AH cleans himself up~~~
Of course this is all based on if hes actually telling the truth on the amount of oxy he is using, you can never be sure with us junkies. I heard a term once when i was in rehab back in December "How can you tell when an addict is lying?? His lips are moving." Hated the saying at the time but now that ive been clean for a little while that saying is deaaad on! lol