I had an injury once through a noob and I was angry but didn't do anything. He was in the same league as me and we had a game 10 hours later. Guess it was an accident and he just wanted to test out his tactics...I don't know...But after that it never happened again. Don't know what he was thinking but since there's no interaction features I will never find out.
The time I was more angry was waking up 20 minutes before the match and see my players are all tired because a Japanese dude had a friendly. He plays me everyday and from losing big into ties now, since there's no injuries, I don't mind, let the dude practice his tactics, whatever but half hour before the game was unacceptable. I unfriended him and the dude requested again. I rejected few times but the dude was consistent so I added him again and he never played a friendly again...I felt so bad I had to play him myself LOL.
So in the end, if your friend don't know how to look at your schedule and see when's the right time to test out his new tactics and play with easy tackle then unfriend him. I had a dude unfriend me once because he was at higher level and had the exact same formation as my coming opponent. I picked the perfect time for him to rest and in the end he won 3-2 with no injuries and 12 hours before his next game....After what the hell you guys add friends for? To exchange gifts? You only need 4 friends! Filter out the sneaky dogs who might be your opponents friends and say sorry to the noobs tell them byebye. I rarely arrange friendlies but after got knocked out the cup I don't know what to do with my well rested squad so I played friendlies and no1 complains even though I rip them apart. Friendlies are good to adjust your tactics to its best. They are more useful than coming on this forum and posting Help topics.