Family law

1, If due to dispute as you say here they have voluntarily living apart then “desertion” rounds he cannot use to file for divorce.
2. He should have earlier filed RCR and let is to slow paced all these years.
3. Now the grounds that is most suitable for him is mutual consent divorce as 4 years under dispute parties are mutually living apart which is pointing to mutual consent divorce approach and not divorce under one of the available grounds of Hindu marriage laws is my view.
4. I see some hiccups here so it is upto your friend how he tackles the issue of mutual consent which is preferred over any other grounds so that S. 498a / 406 or DV is not counter blast.

That part he has to tackle otherwise also to answer straight just after your friend after 4 years files a divorce then it is obvious the other party will not send French kiss as reply but may retaliate by usage of some adversial law which can be RCR or DV or S. 498a / 406 or who knows she may even agree so speculation should be avoided unless affected party themselves tells whole brief here and then proper remedies comes as reply and in such absence above reply will do to speculate more by you who is a third party here !.