Perhaps I should explain my joke 'ever the optimist' in hoping for theft only as a conviction for B.LOL Only a law student could think that possible. In law, it would mean that B knew nothing of the violence or any attempt at robbery by A or anyone else and had chanced, in all innocence, upon the victim's money lying in the street and , not knowing how it got there, had decided , dishonestly, to take it. Well now, that is so far from the evidence as to be academic.It could not be argued. Some students might wrongly think too that A had done all the violence and, in some way, B could only be guilty of theft, though A was a robber, since B had not wished the violence (if so found by the jury).
No, the robbery has not been completed until the money is taken, appropriated, since, up to then there has been no theft. Someone is guilty of robbery if 'he steals and immediately before or at the time of doing so , and in order to do so, he uses force on any person [or seeks to put the person in fear etc]'. No theft, no robbery. B has joined in before the robbery is complete and has helped A 'finish the job'. He may be a reluctant participant but he's a participant and he knows what is going on.
And, of course, being drunk would be no defence. A drunken intent is still an intent. That the man would not have so acted had he been sober is no defence. Nor is it material that A asked B to get the money. B would only escape if he was under such duress from A , in reality in such real fear for his life that his will was completely overborn and even running away was not an option. .