The protection spell will prevent him from being zapped too terribly when reaching through, but he'll still receive a sharp sting that he might feel for the next few minutes.

For their efforts, they're able to retrieve a box containing a letter. Curiously, it appears blank at first glance, but Istredd will be able to determine that the writing has been concealed with magic ink.

They can either take the letter with them and slip out of the room quickly, or they can attempt to examine the letter in the room and put everything back as it was before they leave. The latter will take more time, so they can weigh the chance that whoever has this room probably won't return tonight...if they're sure about it. The room doesn't appear to have been touched all night.

You can investigate the letter more closely here.