After granting evisa my friend not allowed in Bahrain by i'mmigration

That is exactly what I and khalidkhanblog have said i.e. "The decision to let the individual enter is at the discretion of an immigration officer".

No one can say what the exact reasons for refusal of entry were.  And in the eyes of the law, your guarantee is meaningless unless YOU are sponsoring his visa; either as a family member or through a company - that is why sponsored visas cost more (if you ask someone to sponsor you) and have lower rejection rate; because the sponsor can be held accountable. 

If I had to assume what happened - based on incidents that have happened to others in the past:

-Person from the sub-continent fitting a certain profile lands i.e. below 40, not affluent or educated (blue collar / labour) and with no history of travel for leisure
-Attracts attention and on questioning is not able to explain purpose of visit clearly
-Has unsponsored visa i.e. does not have a visa from a sponsor or guarantor
-Based on the profile there is a risk of overstay and working illegally.  Lots of these cases in Bahrain from the sub-continent
-Person is asked for a contact in Bahrain who he is going to meet or stay with
-Contact is called to cross verify story.  Even the smallest inconsistency between the two stories creates doubt e.g. length of stay, how you know each other, what are you going to do during your stay etc etc
-If there is the slightest doubt, entry is refused

I always advise people in these cases to (and especially if they are from the sub-continent, not very educated, without history of travel for pleasure and not visiting family - generally when they are going to western countries but advice also applies here):

-Dress and be well groomed
-Answer immigration questions truthfully and to the point - not more not less.  Know answers to basic questions i.e. purpose of visit, how long are you here for, where you staying, when is your return flight, how will you support yourself financially during your stay, what will you do while here, what is your job etc
-Move through the airport with a sense of purpose; don't loiter around trying to call your contacts outside for advice on what to do - in most western airports, they spot suspicious behaviour like this and pull you at immigration for questioning