Their plan was to get back to the castle and inform Professor Dumbledore regardless. He had to be told as soon as possible. Their details may have been only a few but anything that could help, they planned on using. Though it hadn't been confirmed if Draco was, in fact, in association with the Dark Lord, they had every reason to believe so. He wouldn't have been around the psychotic mass murderer Bellatrix Black-Lestrange, unless they had malicious intentions.