Clacking 1999 Tercel

My friend's 99 Tercel started clacking while out on the highway. He limped back home. I haven't heard it but it sounds like a hydraulic lifter or something. Someone suggested that he empty the oil and add a half liter of tranny fluid to the fresh oil. Does that sound like it might help a collapsed lifter? What else could be the trouble, and what else can be suggested?

I would replace the lifter, but the ATF may free it. Stranger things have happened. Give it a while at idle. Don't think driving will hurt it If it's a lifter.