Hello, we had a fight with my opponent today, when he played his 4. Defender Reroll stage, Guardian Stance, thus rerolling my own attack dice, and then he wanted to play Tough Luck to remove my own rerolled

dice that he rerolled. I said he cant do that. Can he????

You're right. Read Tough Luck literally: When an opponent rerolls a die. You did not reroll anything.

(Tough Luck doesn't care whose die is rerolled, just who performed the reroll.)

a1bert with saying "who performed the reroll" you mean who throw tha dice or who trigger the reroll?

When you play Guardian Stance does the defender reroll the attack dices with his own hand or force the attacker to reroll the attack dices with his hand? It is not specified.

The text of Guardian Stance says "Use while an adjacent friendly figure is defending. Reroll 1 or more attack or defense dice"
I think "use" refers to the player defending and after that the word "reroll" is not refered to the player defending in order to do all the rerolls by himself but gives a general order a reroll to take place according to the dices choice of the defender.
In this case the players reroll their own type of dices each one.
If I dont miss somethnig, in all cases of a reroll the player with a type(attacking or defending) of dices throw these dices again, like twin sabers, versatile weaponry, targeting computer etc. I mean its a bit weird if defender reroll the attack dices by himself.
So if the attacker rerolls the attack dices(even if he is forced to) he is "an opponent rerolls a die" and I think tough luck is valid to play.

In the other case that the defender rerolls the attack die If we approach " When an opponent rerolls a die" less literally, there is still a chance a die rerolled from the attack pool of the opponent to considered as opponent's rerolled dice, independently of the hand throw it.
I am not sure if FFG wanted to give empshasis to who performed the reroll.

Who physically performs the reroll, regardless of whose die it is and regardless of who has the reroll ability.

There are abilities that force your opponent to reroll (for example Raider on a defense die), and Tough Luck can be used on those. And there are rerolls that you perform on the opponent's dice.

This distinction didn't have an effect before Tough Luck, but now it seems to matter. (Whether FFG intended it or not.)

Guardian Stance says "reroll 1", and when not specified otherwise, the figure with the ability performs the ability. Raider says: "The player who rolled the die must reroll that die".

Actually I'd assume the opposite. Never touch the opponent's dice. With "you reroll the opponent's dice" I would always have assumed, that this means, that your opponent has to reroll that dice. I'd like that just for legal reasons. There are countries, where it's even illegal to touch your opponent's dice.

But I have to admit, that Read As Written a1bert is correct.

I am with A1bert on that.

Note, that step 3. in RRG clearly states

"- Each die may be rerolled only once per attack regardless of who is rerolling the die.",

which clealy means, that both players can reroll a die.

It doesn't matter who physically rerrols the die, what is important is, who rerolled it rule-wise.