Connie, it's a fine line to draw when you are trying to get a friend to do better but don't want to offend them or make them defensive (then they won't listen anyways). How about this...

Tell your roommate that since you don't have a lot of turtle experience you went online to look up "stuff" so you could take care of the turtle for him while you were turtlesitting. Then go over some stuff you printed out, things you learned or websites you found. If he seems receptive, then you'll likely get your point across. If he seems defensive or disinterested, then you may have to get a bit stronger in your position that they aren't caring for this animal correctly, it's suffering and something needs to be done for it.