It's a huge strain on any relationship, friends or couples.
You're basically asking someone to forfeit all tenant rights in favor of a situation where they may even end up being compelled to share the burden's of property ownership. It's a power dynamic where you're actively profiting off someone you consider close to you and it's very difficult not to end up exploiting that power even just accidentally.
This kind of business arrangement always goes way beyond financial advice. It's a relationship problem where you really need to find ways to tip the scales back. You can't just strive for financial parity while ignoring the fact that you're not offering the same services/assurances as a normal landlord. You need to put relationship parity first and foremost.