First of all as he is working abroad, let him check up if there are any institutions available there suitable to him to pursue his career. Otherwise the alternative is to obtain degree from distance learning. So he has to check up which courses or which institutions are recognised there a equivalents and accepted for jobs or promotions. As he had already ruled out AMIE and "is much interested in management but working in the engineering section", let him check up the career prospects and take up an enabling general degree course in science, arts or commerce, or business administration. There are many Indian universities giving degree by distance learning mode. IGNOU is also suitable.He may also try if any foreign institution is giving management degree by distance eucation which is recognise at your place.
What are the various options available after having completed his diploma(Engineering) after 12th or HSC?
After diploma one can join B.Tech by the lateral entry scheme . One can also join for general degree courses. Taking up graduate courses by distance learning is also possible.
Is there any approved and recognized correspondence or distance education from where he can enrol and complete his graduation?
Yes.IGNOU and various universities in Indi are giving this facility.
According to you, which is the best way of completing his graduation and why?
As he is working distance education will be most suitable.
Is enrolling for foreign universities through online study or fast-track mode a good option/choice and are the certificates valid everywhere?
This has to be decided after checking various points like acceptability of the course in the place of working, the mode and expenses etc. It is suggested to visit the web sites of such institutions.
Name some AICTE/UGC recognized/approved universities in India
to enroll for further studies?
There are many universities and IGNOU too. All major universities are recognised. You may visit the web site of UGC and AICTE too for latest information.
Any other points to be kept in mind while opting for such courses/colleges/universities for graduation or further studies?
There are not much issues when enrolled in a recognised university in India. One should only monitor the information and announcements coming intheir websites regarding admission and other details