Possibly. Though the given scenario has the victim engaging a stronger than normal mob. How quickly can they escape combat, or do they fight while taking the damage from both? 30 seconds is basically the same as the DF timer when it pops, so we're assuming the attacker is spotted as soon as they appear. (I find that I have to run immediately if I need to be out of combat to switch and join the DF.) Remember that the hidden combatant gets to choose where and when the fight starts, since they are immune until then. Further, anyone who would go for the cheap kills will likely be overpowered to begin with, so yes, auto-attacks are likely to do the job.

There's also something like five seconds after combat before they can escape by switching back to a non-combat class, which won't really amount to much though, will it?