It seems that Morinaga Milk is the author to go with, just as I thought. Girl Friends was the first series I thought of that I could use and you all only strengthen my opinion.
Yuri gag mangas aren't usually serious enough to really go anywhere and both Marimite and Kannazuki no Miko focus more on friendship and action respectively than on yuri.
Hanjuku Joshi seems like a pretty good choice as well.
Well, since most of you recommended either Girl Friends or Morinaga Milk in general, I'll go with that.
If anyone's interested, I can post whether he started to incline towards yuri or not after reading the series.
Thanks again for everyone's opinions!