The K-books shop where the cels are in Ikebukuro 
was undergoing rennovations when I was there last 
week. I don't know when it is set to reopen, s 
you should check their website before making the 
excursion out there (unless your friend also 
likes doujinshi, Ikebukuro is THE place for girl-
oriented doujinshi.)  

As stated previously, Nakano broadway mall 
(nakano stop on the JR chuo line) is probably the 
best concentration, has 3 stores that I know of.  
Mandarake Shibuya store also has some cels. I am 
told there also a couple cel stores in Akihabara, 
but I didn't run into them when I was there.  Try 
doing a google on cels stores, and akihabara.