a sidebar wiget with login for ONLY that widget

hello,

i am finished with my site, but my customer wants to be able to add info website like when he is on holiday.
i was thinking maybe there is a plugin that lets him login on that widget and only be able to edit that widget. is this possible?
creating a new user with admin rights is a risk as he is not good with computers. or is that the only option??

thank you!