Toyota of Glendora misdiagnosed problems, Glendora California

I brought my 08 scion XB in for a major tune up (90.000 mi.) the sales associate advised me my water pump is in the yellow and needed to be replaced but could wait until the next visit .  A day later I started hearing a squaking noise coming from the serpentine belt area which I never heard before until after that visit. I brought it back at which point the same sales associate told me it was my serpentine belt that needed to be replaced.  I said I would go elsewhere because I did not trust them.  A couple days after that the belt completely shredded and broke off as I was driving on the freeway.  I got it towed back to my house at which point my friend who was an auto mechanic  took a look.  He determined that the belt itself was still in good condition and the cause of it fraying and eventual breaking of the belt  was caused by the pulley bending because of the water pump busting.  Therefore the sales associate and mechanic misdiagnosed the problem which if I had done the replacement of the belt it would have been pointless because the main problem was the water pump.  When I contacted their consumer replations dept. a rude woman named Lila Salsman with her condescending tone towards me just made me want to not deal with them at all, she then blind transfered me to the manager Dave Davey and he told me to bring it in.  At that point did they think I would want to even set foot in that place again so they can take advantage of me?.  I know what you are thinking that maybe just maybe out of coincedence that the water pump broke right after they offered to fix it or maybe even tampered with it?  I had no way of proving anything.  My main complaint is that they misdiagnosed the problem.