This issue has been resolved. Feel free to spin your friends around and around.
We're aware that when you summon a friend's Archon in a dungeon that the attack animation does not end properly. We are looking into the problem will update this thread when we have a fix for it. In the meantime, you unfortunately will not be able to summon a friend's Archon, but you can still take him for a spin in the event dungeon!
Joel | Community Manager | Boss Fight Entertainment