Basically agree with this. It will depend on the doctor but those are the options I would suggest are best suited. Obviously it's better to avoid a bupe script if possible as it's a step up (or even several steps up) the 'opi ladder', but it would almost certainly come with the support of a key worker and basically the same support as you'd get for an addiction to heroin or anything else. Tapering would perhaps be a less 'dramatic' step and almost certainly be the first option offered and worth trying as going onto an opioid maintenance script is a big step. I doubt that methadone would even be considered but, as has been pointed out and you've said yourself, it really shouldn't be an option in anything other than the very most extreme of cases.
Tapering ideally, bupe is perhaps not a bad idea if tapering simply isn't possible. And good luck to the both of you - your g/f (obviously) and also yourself cos she'll need your support cos any addiction is difficult to deal with and we could all do with all the help and support we can get