I'm a completely new DM of a group of completely new players. My party of 6 level-3 PCs split up into 3 groups of 2 in a dungeon. One of PCs decided to just throw open a door behind which was 3 enemies. The PC that opened the door rolled highest initiative and immediately cast invisibility on himself and ran away. The enemies turn was next and the only option I could conceive as realistic was the enemies mobbing the only remaining PC (Low AC wizard) that they could see, who had played no part in the fact he was now under attack from 3 enemies that can attack twice each turn.

I had to fudge rolls to stop him dying instantly from falling unconscious and the remaining 3 attacks immediately failing his death saves.

Questions to more experienced players/DMs:

1. Is there anything else the enemies could've conceivably done, like swing around for the PC that just went invisible in front of them? I felt at the time the only conceivable move for them was attack the only PC they can see.
2. Is fudging the rolls the best thing to do here to save an innocent and completely new player from their first PC death being an instant 100-dead due to another players actions?
3. This isn't the first time this player's actions have been against other members of the party. He has also attacked an NPC (a cleric who worships a different god to the PC, which he hates) which played a part in another characters bond/goal (funnily enough the same PC who he nearly killed above), and again I had to break the rules to keep this NPC alive. His justification each time is his character is selfish. Is this acceptable behaviour seeing as it is technically within character? The player that is on the receiving end of his actions is getting frustrated.