No you don't have to inform them. You should comply with the wishes of the Donor, I presume as he is signing his own Power of Attourney then he is capable of making such a decision. If he doesn't mind either way, which will cause the least problem, telling them now (are they likely to kick up a fuss and upset the Donor??) or telling them or having them find out when you start to make the decisions?
Is it the financial kind or the all embracing kind and (don't answer this but think about it) do you benefit from the will? If you don't then less probs I would think