Can one quit taking oxycodone on his/her own?

A friend of mine’s wife has been on oxycodone for at least 7 years because she had a work-related accident. She has been taking it non-stop 12 – 15 pills a day, although she’s supposed to take only 3 a day. She sleeps all the time and whenever she tries to be off the pills, she’s hell on the earth. She smokes like 2 packs of cigarettes a day too. She is down 3 pills a day now again, trying to cut back and eventually stop. I’ve heard that quitting the pills altogether can be very dangerous… and the person needs to go to rehab. She’s trying to quit on her own… is that possible? What could be the withdrawals symptoms? Thank you!