A) I think you should take everything that individual said to you with a grain of salt, based on the few things you've told us they said.

B) Your snake looks thin for its size. It can easily eat an adult mouse, based on your photo. I would give it an adult mouse, particularly if that's what it's used to. It looks like it needs fattening up.

C) Put the live mouse in the cage with the snake. Do not lift the hide, or disturb the snake, just put the mouse in. The snake will stalk and kill the mouse, and then eat it. I recommend keeping the area quiet and preventing a lot of motion outside of the cage while the snake is eating--they can be very shy, and kill but then refuse to swallow prey if there is too much activity. (They want to keep their jaws free in case they need to defend themselves--they are very vulnerable when swallowing).