Herion isn't even close to being the "strongest" of all opiates. In fact, in the world of opiates, heroin is only about 2-5 times more potent than morphine. In comparison, Fentanyl is 100 times the potency of morphine. Sufentanil, used in OR settings, is 1000 times as potent as morphine. I'm not downplaying heroin's dangers, just pointing out facts.
Certain people who have addictive personalities can get addicted to a drug like heroin very quickly. Since you said she had an alcohol problem it makes that possiblity more likely. Heroin is the strongest of all opiates. Most people here are addict to vicodin and don't know how addictive and strong it is. Pot is rarely a good idea and especially for an addictive person. Naltrexone is used for a prevenitive agent, to cut cravings but more as a deterent. If you take any opiate on it, it won't work. If you take naltrexone and have an opiate habit you will get very ill. I definatly would suggest suboxone or methadone as they are supposed to be treatments of last resort or "harm reduction therapy" All the best