Did you look it up? At first glance..this is what I found

used medically as an analgesic and in the treatment of narcotic addiction

It also is used to prevent withdrawal symptoms in patients who were addicted to opiate drugs and are enrolled in treatment programs in order to stop taking or continue not taking the drugs.

is she perhaps seeking treatment?

Google up "will methadone make me high" and read some drug user forums on it, they say it will definately get you high, they tell you to take only so much and wait for at least 3 hours becuase it sneaks up on you, not a rush like oxy's or hydro's. People od from it because it doesn't work fast enough and they bump another before the first kicks into full affect.

She may be using it to help herself, but I highly doubt it.

Methodone and Suboxone are opiates that are generally used to wean people off Oxy and other opiates but which can be abused. When their oxy supplier(s) are gone or they are out of money and panic, they may go to methodone or suboxone because it's better than nothing in their mind. It could be a good thing - like if she can stay on it for a short time and limit her use even if that wasn't her intention. I think the idea behind methadone clinics is that it's better to have opiate addicts doing a legal opiate than dealing, stealing, etc.. I personally don't see it as a long term answer and prefer people do detox and rehab and get off opiates altogether.

Some people will never be drug free, so some will always be on methadone, or any number of drugs that let the person have a high. I know some block the receptors so that the person cannot get the same kind of high.
She is an addict and I hope she doesn't kill herself. Sometimes its better we not know what they are doing.