I have a question: Have you seen the inside of it already?
Many dojo's are very plain and some are meant to be. I'd consider a nice plant... a hardy green fern of some kind might be nice.
Does your friend practice Aikido specifically? If so, and if you like the 'print' idea, I'd check out:
There are many items under the "New Products" section with prints, shirts, pendants, etc. Anything from there would be very nice and welcome. Depending on how good of a friend he is, there is a variety of prices of items there to select from. I'd avoid giving any uniform or Hakama as these are generally very personal, though a gift certificate would be okay.
But getting back to the Dojo itself, I think a plant of some kind would bring some life to the place. Ficus trees are nice and do well indoors. Use your best judgement and see if you can find some of your co-workers students for ideas maybe?
PS: Give it a shot! Martial arts are fun and Aikido especially! Any 'co-worker' who cares enough to want to help a friend's new dojo must have a pretty good person in that friend! Join in and have fun!