Moral dilemma

One of the groups at work today left a $20 tip for each of the 4 guides who did their programs. 3 of those guides were still here at that point (it was a few hours after they had left, the administrator came back) so I gave those 3 guides their tips, but the 4th was gone and will not be back until next Tuesday. My question: do I give her all $20, give her maybe $10 and keep $10 for myself, or keep it all? Obviously the correct answer is to give her all $20, but what would you do?