Can Ward Slivers protection be responded to?
Your friend is incorrect.  The "magic words" for a triggered ability, which an ETB is a subset of, are "When", "Whenever" and "At."  Triggered abilities use the stack and can be responded to.


  603.1. Triggered abilities have a trigger condition and an effect. They are written as "[Trigger condition], [effect]," and begin with the word "when," "whenever," or "at." They can also be expressed as "[When/Whenever/At] [trigger event], [effect]."


"As X enters play" indicates what is known as a replacement effect.  It changes the standard rules for what happens when the spell resolves.  (In addition to it becoming a creature permanent as per standard rules, you also choose a color.)  Such an effect does not use the stack.


  614.1c Effects that read “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” or “[This permanent] enters the battlefield as . . . “ are replacement effects.