One of the mandatories a couple has to do before moving in together is to put their financials on the table and create a budget. If she's living paycheck to paycheck but can still afford to fund half of a domestic situation with you that's one thing. But if her intention is to go back to school and have you support her that's clearly a nonstarter. So you'll have to sit down and talk about this, including every detail of her debt to income ratio and how she'd be able to pull her own weight. How to "broach the subject" is to just tell her that living together is a business arrangement as much as a step forward in the relationship. It's simply not something you can do without knowing for certain that she'll be able to pay her fair share of everything in the household.