In my opinion, you should train for it during sparring, if you don't it will probably not be effective in either tournaments or in real life situations. You or the students will not have confidence using this skill...and when it comes time, you use what you know best.
What happened with your friend was unfortunate and you can't or shouldn't train based on fear, lots of things could have contributed to his knee going out, including genetics, and lack of skill applying this technique. What you can do is apply steps to ensure a couple of things, make sure students are properly warmed up and include lots of stretching and flexibility drills. Two: work the techniques at slower controlled speeds and build up from there, covering the potential injury factors with the students and three talk to them about how to protect themselves using martial techniques, foot work, cat stance, and or break falls and the like.
"if its ok for shaolin wuseng to break his vow then its ok for me to sneak behind your house at 3 in the morning and bang your dog if buddha is in your heart then its ok"-Bawang
"I get what you have said in the past, but we are not intuitive fighters. As instinctive fighters, we can chuck spears and claw and bite. We are not instinctively god at punching or kicking."-Drake
"Princess? LMAO hammer you are such a pr^t"-Frost