Your question and scenario are 2 different things. On what you should before launching a company: (1) It must be a passion (2) Have finances. (3) Understand 80% chances you are likely to fail.
That's said, on the scenario, let him/her consult a lawyer. If your friend is going to have these worries, he/she is better off not starting.
And unless, it's a big idea or stealing the employer's clients (e.g law firm), the employer is unlikely to care about chasing after him/her.