My friend recently gave birth to a baby boy. Both mother and father has B + blood group but child has B - group. Can u explain?

Yes, there are many factors it is not as simple as that. It is not only the antigens but also the anti bodies.

Blood Types | American Red Cross (http://www.redcrossblood.org/learn-about-blood/blood-types)



Blood Type Chart - Human Blood Typing - ABO (http://www.vaughns-1-pagers.com/medicine/blood-type.htm)