English Law "Secondary liability"

Peter and Donald are best friends. Whilst out drinking one night Peter sees Derrick who owes Peter money. Peter tells Donald that Derrick is here and that he wants to get his money back.Peter says "I'll just give a little slap and maybe the threat will work". When Derrick heads to the toilets Donald grabs him from behind and puts him in a headlock. Peter takes out a knife and waves it in front of Derrick's face. Derricks says "don't be stupid I've got the money, It's just gonna take a few days". Donald, still holding Derrick, says "let's not be silly, pay up now or face the consequences". Derrick says he can't at which point Peter loses his temper and stabs Derrick repeatedly.Both Donald and Peter run away but are tracked down later by the police. In police interview Peter admits to murdering Derrick. Donald refuses to accept any responsibility.

Advice Donald on his liability as regards secondary participation only.