I think one of the keys is to get a rental agreement, and establish that she's a tenant in your condo rather then having a loose agreement in which she could imply that she was assisting you in paying a portion of your mortgage.
A trip to the lawyer would give you a good idea of what is nessesary.
That said if you're hesitant for reasons beyond the legal aspects then that should be a red flag to you that you're not ready.