Will commercially produced soap coated in epoxy resin last?
Once epoxy resin is cured, it's extremely stable and resilient against mechanical and chemical damage. It's impossible to dissolve cured resin in water, so your soap is well protected.

Adding water to resin before it's cured is a problem, though, and probably the reason why the person told you it's impossible. As a rule of thumb, if you add other substances to the epoxy resin (like paint, ink or other water solutions), you cannot add more than 10% of the epoxi's volume. If you add too much other stuff, the epoxy will never cure completely and stay soft or bendable, tacky or even runny like honey forever.