I have to agree with everyone on here...."as prescribed" is whats necessary to not overdose!
I'm so sad for your "friend". It sounds as though you think this friend may harm themselves. It sounds like an awful situation. My condolences to you and your friend. Have your friend take the medication as prescribed.
Take as prescribed. TKing extra will only build Tolerance, requiring more and more for the same affect. More, usually, is not better, even if they are not working as good as one would like. Ask the doctor what max dose is at this time is, don't administer more.
I'm sorry for your friend, honey.
We aren't doctors, and you're asking a very serious question. There is no cut and dry answer when it comes to opiates; so much much depends on the person, body weight, general health, and of course, how much dependency already exists.
Your pal has cancer; I assume these meds were prescribed by their oncologist. That's who you need to be asking about this.
Good luck..