Instead of asking for money, the LW should suggest they divide where they spend their time together: half at his place, half at hers. Then he can rent out his home on AirBnB while he's staying at her condo. If the girlfriend objects, the LW can point out that it's unfair for her to take all the profit opportunities stemming from their cohabitation.

This assumes the LW is willing to put his place on AirBnB. If he isn't, he should just forget the whole thing. Renting one's home to visitors involves risk and some work: cleaning, coordinating with renters, etc. If the LW is unwilling to put in the work and assume the risks, then his girlfriend's AirBnB operation isn't costing him anything.