Neville did have the information to justify his actions.
Harry, Ron, and Hermione are clearly up to no good:
"... if Filch spots one of our feet wandering along on its own —"
"What are you doing?" ... Neville appeared from behind an armchair ...
"Nothing, Neville, nothing," said Harry, hurriedly putting the cloak behind his back.
Neville stared at their guilty faces.
"You're going out again," he said.
"No, no, no," said Hermione. "No, we're not. Why don't you go back to bed, Neville?"
And the last time the trio were up to no good, they not only lost their House a lot of points, but their lies got Neville in big trouble too when he tried to help them:
"Harry!" Neville burst out, the moment he saw the other two. "I was trying to find you to warn you, I heard Malfoy saying he was going to catch you, he said you had a drag —"
Harry shook his head violently to shut Neville up, but Professor McGonagall had seen. ...
"I think I've got a good idea of what's been going on," said Professor McGonagall. "It doesn't take a genius to work it out. You fed Draco Malfoy some cock-and-bull story about a dragon. I suppose you think it's funny that Longbottom here heard the story and believed it, too?"
Harry caught Neville's eye and tried to tell him without words that this wasn't true, because Neville was looking stunned and hurt. Poor, blundering Neville — Harry knew what it must have cost him to try and find them in the dark, to warn them.
"... All three of you will receive detentions — yes, you too Mr. Longbottom, ... and fity points will be taken from Gryffindor ... Fifty points each,"
At this point, it would be completely unreasonable to expect Neville to just take their word that they have good reason to be sneaking out again.
It's admirable because Neville has something to lose
Standing in the way of his friends' troublemaking — especially as Ron starts turning violent — could easily have cost him his friendships with the trio and their supporters, but nobody would have blamed him of anything if he turned a blind eye. But he did it anyways, because it was the right thing to do.