2008 Toyota RAV4 Put in diseal fuel by mistake

My 16 year old daughter filled up her tank with diesal by mistake, it was on empty. She got about 50 feet and then would not start. Had it towed to my local mechanic (not toyota dealership). He drained the fuel tank (did not drop it) and flushed it out. It is running fine, but a little sluggish than the normal 4 cyl. What should I expect now, my mechanic says run it through two tanks and then if still sluggish will replace spark plugs. What about my toyota warranty, can they find out I have this happening. Is there long term effects to this, or if all is good after the plugs are we in the clear.