So you invite some friends over to your place to ride. This is your own farm, not a riding school/business just friends getting together to ride and have fun. You furnish the horses and tack, match the riders experience to the appropriate horse and start off riding. Everything is going great, everyone is happy you ask several times if each rider and their horse is getting along fine or any issues, alls good everyone else replies. The rider with the least experience in your group, their horse kicks out at another horse that came up behind them and makes contact with another riders horse and ultimately kicks that rider in the leg as well. (Now the horse who kicked out, normally is very easy going and you can pony other horses and is a great beginners horse, so the bad attitude is totally NOT normal). The rider who gets kicked, is hurt with a gash in leg. You get them to hospital to check for fractures or broken bones. No broken bones, just very bad bruising and a gash. Who is responsible? Is it a ride at your own risk?
(Also, the rider that got kicked, is an experienced rider, having as much experience as you or possibly more experience. And they know the horses, rode them many times before and knows their personalities.)