Stealing a "play at any time" item as a thief in Munchkin?
You say the thief went to steal the item, but here's no such game mechanic. Either the thief has stolen the item, or they haven't. Stealing is not something that has a duration, so you can't "interrupt" it. There's no mechanism for time travel or precognisence in Munchkin, so an action (e.g. stealing) must be completed before another one can be started.

If the thief hasn't stolen the item, the owner original (and current) owner can play it.

If the thief has stolen the item, the owner original owner can't play it since they no longer own it.