You should both read Lend to Live: Earn Hassle-free Passive Income in Real Estate with Private Money Lending. It's published by BiggerPockets and goes in depth on the do's and don't's of private money lending from the lender POV. If you want to protect your friend's capital as if it were your own, you'd want to:
1) Secure the loan with real estate in the form of a deed of trust or mortgage (depending on the state the property is in)
2) Obtain property insurance binder with your lender as an additional insured
3) Obtain a lender's title insurance policy protecting them (different than an owner's policy) and transfer all funds through an escrow closer or real estate attorney (depending on the state, the requirements for closing may differ)
4) Use a real estate attorney who also practices private lender law in the state the property is located in. Don't utilize a title company to create these documents and definitely don't google for any doc templates on the internet. That's not super thorough or safe for either of you.
5) Use a third party loan servicer to make your monthly payment collections easier as well as the end of year tax documents needed. Time saver for the both of you.
That's just the tip of the iceberg but a small snippet of tips that you can get out of this book.
Good luck and LMK if you have additional questions! I'm one of the authors of the book and own and operate a PML business with my husband :)