My tamed wolves keep killing my friend multiple times!
Sounds like the game rule forgiveDeadPlayers is set to false. Try changing it to true, then your wolves should stop attacking your friend when he respawns.
Edit: Nevermind, that game rule didn't exist back then. If you didn't update to 1.16, there's nothing you can do about angered mobs ignoring death and respawn. 1.15 and earlier just does that.
You can stop this from happening by having your friend relog. Aka get them to get off then back on. Then don't have your friend harm your Minecraft character.