Is One Week Friends manga still on going?
No, it is still ongoing according to Wikipedia. At top right of the page under "Original run", it says "January 21, 2012 – ongoing". 

Sometimes, Wikipedia doesn't update the information or the status is left like that when the series is on a long unexplained hiatus. Just to be sure, in the Volumes section of that page, it says that the release date for Volume 7 is April 22, 2015, which shows that the next volumes is in the works.

Also, Manga-Updates says the same thing under "Status in Country of Origin" (6 Volumes (Ongoing))

According to a post on Anime News Network provided by Torisuda, Volume 7 is the last one.


  The sixth compiled volume of Matcha Hazuki's Isshūkan Friends. (One Week Friends) manga is announcing on Tuesday (26/07/2014) that the seventh and final volume of the manga will be released in Japan on April 22, 2015.