Dung would have apparated there after receiving Stu's message, and also managed to apparate outside the wards, but still got hit with that bit of Crucio. He didn't bother with a mask, after almost getting killed by Rabbit, he knows they know him.
He helped take down the wards as fast as they all moved, and went after the wolves first, helping Stu distract them long enough to get Ed out of there. After they were killed, he would've followed Stu to Ed's house, and probably just paced outside and smoked a lot.