My friend got married to us citizen but things are not working out between them. He had not filed any aos or any petition on behalf of her and he now wants to get seperated. Anyone has any idea about this?

Depend on your friend status if he is us citizen it is does not matter .tra keti bata green card banako vaho he need to wait till he will be become us citizen

Haina he is student, martied in good faith...tara pachi kt le rang badali, so things didnt work out, now they dont live together...so he wants to know where to begin because this all happened all of sudden...

 If your friend starts the divorce process, he might need pay alimony, so let his wife to come with divorce plan,

 since he never filed AOS, there's no problem for  now. But in future it might look like a red flag, if he gets married again and try to file AOS on behalf from his wife.  I don't think it matter who files for the divorcee from immigration POV. 

Best option is to try to make things work now than regret later. 

Dont stressed out .. Just find a lawer who does unconditionl divorce u can find in craiglist or yellow book.. They will charge u around 700 ... And u both sign the papper and the lawere will file in court .. It will be final in 45 -60 days .. .. So dont stressed out .. Dont worry abt alimoney coz u should be mRried for 5 years to claim that .