The thing is, you're not looking at this as a job, or an investment, or a profit-generating enterprise. You have a friend who is staying with you, and helping you with the bills. I don't see any ethical issues at all with this.

I actually find myself in a similar situation, as my friend just left her partner, and, well, I had a spare room. She kicks me a little money to help with the bills, and I keep a roof over her head until she has somewhere better to go. In my mind, that's different than looking at landlording as a job, or worse, an investment to generate passive income.

What I'm saying is that intent matters.