When you take on a boarder (or a renter) you put yourself in a position of responsibility. Sometimes it isn't about a BFF turning on you, its just that you made a simple mistake--forgot to close a gate, etc. and you are honest to goodness simply responsible for the consequences. None of us are perfect. Also keep in mind you don't only need to protect yourself from liability from your tenant/boarder. Your boarder will invariably at some point or another have guests out to visit/ride her horse.
I have found that being a landlord is easier and more profitable than running a boarding business. My advice would be to consider renting the MIL but be very limited about how many boarders you take on. You will most likely be losing money in the short and long run to put up a new barn barn for boarders. Your insurance agent should be able to advise you about proper coverage for a single boarder--the coverage you need may not be terribly expensive.
My post was totally Debbie Downer (I still sound a *tad* bitter 10 years later--oh well) but I actually WOULD consider having the right friend board with me. It's nice to have someone to ride with or a trusted person to keep an eye on things when you are gone. Just spell stuff out. I've boarded with friends and had we done a better job of communicating up front about expectations, I think we would have done things differently. We are still friends though!
I would make sure I covered my bases. Have a good contract. Sign waivers if that's the recommended thing in your area. Post the liability signs for your state. Have the right insurance in place. Take all the precautions you can--that's the point I meant to make.
From personal experience I would be very careful about the friend's horse. I know my mom's rental does not include renter's insurance (for their stuff) so she has them sign that they understand this. She cannot charge dime one for horse board. She has had many people "board" for free, which helped, (they did all the work), and,with them signing all the "hold the landower blameless" paperwork crap, I think it helped. And boarder's friends riding there is a no, as per her insurance. They would all have to sign waivers, wear helmets and all that which she woudl have been happy to do but was not always there. Definitely talk to your agent. Maybe some kind of umbrella policy would work for your needs