He also can't say to her face, BECAUSE Dendi trusted them both (that its no offensive blah blah blah), he ended up in big trouble and Dendi SHOULD NOT trust strangers (??? they probably hang out prior to that) and always double or triple check everything he's asked to do.
Lastly, i find amusing that this girl puts Dendi in guilt for trusting her and her friend. "We are all have a little bit guilty" indicates princess syndrome, when your actions and responsibility is shared between everyone involved and never the perpetrator; textbook girlish behavior and immaturity.
And yes, Dendi is stupid for trusting people, as shitty as it sounds.