In this case, dream doesn't mean the thoughts and visions you have while asleep, but the thoughts you have while awake:
2: an experience of waking life having the characteristics of a dream: as
a : a visionary creation of the imagination : daydream
A daydream is
: pleasant thoughts about your life or future that you have while you are awake
The action of daydreaming is
Daydreaming is a short-term detachment from one's immediate surroundings, during which a person's contact with reality is blurred and partially substituted by a visionary fantasy, especially one of happy, pleasant thoughts, hopes or ambitions, imagined as coming to pass, and experienced while awake.
So, in simple terms, daydreaming is when you have thoughts about happy things or your goals in life while you are bored, usually at work or school. I'm sure you are familiar with such a scene.
So, in the script, the friend is saying that if the girl gets a job and it was boring, then she would have something to dream/daydream about. In other words, she can think about becoming an actor while she is bored at her job (if she gets a day job, and pursues acting).