My daughter is having slight dry cough & nose block problem. is diphenhydramine hydrochloride a good medicine for this problem?

Maybe, maybe not: Diphenhydramine (benedryl) is and antihistamine. That means it blocks a pathway involving a chemical called histamine, which is released especially with allergy issues. Thus, benedryl with decrease congestion, but only congestion caused by histamine. Cold/respiratory infection congestion is not caused this way and thus wont be blocked by this medication well. There are other options...