Default Is it possible to flash from Boost to anything else? Sprint maybe?

Hi, My friend Joined Boost Mobile and they gave him an HTC One. He is not happy with Boost and is going to change carriers. He wants to know if he can flash his phone for another carrier like Sprint or Verizon or ATT or T-mobile or is he better off selling the phone on Ebay and getting a phone through his new carrier? Does this Boost phone even have a sim?? He says it doesn't.
Thanks for any input you can give.