It's clear you care a lot about this person, and really want to help them out. I have to say, from an outsider's perspective, this person needs to do some work to help themselves. Having as many rental properties as you are implying, and being completely financially destroyed by one of them not paying rent for awhile just sounds a bit crazy to me. Your friend needs to cut expenses bigtime and suck it up for a year.
I get that you want to help her out, but the reason there are so many warnings about being overleveraged and isolated to one investment are because these situations are not good. It makes me think of someone who's making big $ on an oil rig and blowing it all on trucks and fishing equipment who gets let go, then finds out they have cancer and let their health insurance lapse because they couldn't afford it. Do I feel bad for them, sure, but they should've saved money during the good times to prevent this from ever becoming an issue. I certainly wouldn't be thinking about cosigning for loans for them or being business partners in any way.
This situation sucks, so she's going to have to do some things that suck for awhile. Take in a roommate, sell some stuff, sell a house, get a job, etc etc. If there's no change in situation expected for a year, putting a property up for sale seems reasonable. Even if it takes 3 months to sell and 3 months to get paid, she's still got 6 months left in this situation where she can use the money. If she keeps going how she is, I assume she's going to not be able to pay the mortgage on a few of these places, and will lose them anyway.