Marguerite -- I lived in Addis for eight months last year, so am happy to help! Truman Kate Otto (cc'ed, I don't think she's on this list yet, but should be!) also spent some time there.
She should try to join the Addis Diplo List:
https://groups.google.com/forum/?hl=en&fromgroups#!topic/addis-diplo-list/3DBppkl6npM And if she uses Couchsurfing, she can also try the Ethiopia group there to find roommates. With both those options I'd say be wary of any 'brokers' who want to act as middlemen: depending on how long she goes for it might be better to arrange something for when she firsts arrives, and then nail down something for longer-term once she's already there, as that would let her figure out where the accommodations are, and how good they actually are.
If you put your friend in touch with me I can answer any questions and put her in touch with the Addis ultimate frisbee group! Best,
Brett