By and large the best way I can think of is a change of environment. A large proportion of addiction is following a familiar routine. This includes people, places etc. which are likely to trigger the trained impulses to use meth. I think your friend needs to take a serious break from their everyday existance. If possible this should certainly include access to the drug. You'd be surprised how much a change of scene can help break the vicious cycles of behaviour which are all too easily follow when the basis of self inflicted entrainment has been well and truly established. They definately need to avoid contact with people who are using meth, regardless of their habits of use. For many the tempation is just too much, especially when they see others 'enjoying' its effects.
If your friend has established a long period of use and has developed a physical dependence then withdrawal is likely to prove an even greater issue than the already painful psychological dependence. I am quite hesitant to suggest the use of other drugs to combat these feelings lest they leave one addiction behind only to begin with another. Nevertheless opiates are probably the best known method for treating amphetamine addiction. I certainly wouldn't recommend anything stronger than codeine or maybe some benzos. This matter should be approached with extreme caution, and if possible perhaps it should be administered by a close friend, family member/housemate etc. Be very careful, however during the period of cravings, states of sheer hopelessness and unthinkable depression are commonplace. IMHO it is certainly preferable that during this time the user might find other ways of alleviating these symptoms rather than returning to meth. I cannot stress how careful one needs to be with these matters & suggest professionals are consulted.
Addiction is often the effect of a deeper cause and without knowing your friend I would hazard a guess that they are combatting something else as well as meth. I would suggest they take every step they can to resolve whatever issues can be identified as the cause for such self-destructive patterns of behaviour. This must be addressed case by case & I don't think there's much else to say with regards to this, save that if there are underlying problems; treatment of the meth addiction will only provide short relief. Counselling is often of great help, I would personally suggest alot of introspection & personal analysis is required of your friend. Ignoring things only makes it worse, which brings me to my final point and that is friendship.
When someone is tackling any form of addiction, especially the consequent period of meth withdrawal, feelings of being completely powerless & hopeless are extremely trying for someone who is often having to face other matters. They can often feel utterly daunted & overwhelmed, it is here that you & other friends have a responsibility to do all they can for someone they love. You're obviously a good friend & I'd suggest providing as much support as you can. Troubles truly are made lighter when they're shared. Avoid situations such as group 'interventions', for in my experience they tend to create feelings of isolation in an already troubled person. Try as best as you can to show how much you care without loading them with guilt. A group presence should express nothing but love & support at such a time.
All of that said, I have absolutely no qualifications in this department and professionals should be consulted first & foremost. However I do think a change of scene is required. Detox Units are always a very good option although somewhat pricey.
Good luck
EDIT: I forgot to mention; I've personally found that a solid course of the amino acid L-Tyrosine works wonders for combatting post meth depression. It is available from the health food store, I'd recommend buying it as a pure compound rather than in tablet form as its alot cheaper. Usually retails for around $25 for 50g.
1800 888 236 - ADCA
Check out these sites for more info:
http://www.adf.org.au/
http://www.drugsafe.org/
http://www.kidshelp.com.au/research/LINKS/drugs.html
http://www.drugs.vic.gov.au/directory.htm
http://www.drugs.vic.gov.au/treatment.htm
www.vauclusehospital.com.au/detox/