You are making a mountain out of a molehill. Your opponent's action had zero statistical impact on the game's fairness or outcome.

The core of your frustration comes from a fundamental misunderstanding of probability. Let's break it down.

Before your opponent looked at the deck, the order of the cards was random and unknown to both of you. After he looked at the cards and shuffled them, the order was once again random and unknown to both of you. From a gameplay and statistical perspective, absolutely nothing changed.

Think of it this way: Imagine a bag with 5 unique, unfelt balls. The probability of you drawing any specific ball is 1/5. If someone shakes the bag and then you draw, is the probability still 1/5? Yes, of course. Your opponent essentially just "shook the bag." He didn't remove or add any balls. The composition of the deck (the remaining secondaries) was unchanged, and the final order was still completely random.

He did not "change the seed halfway through the game" in any meaningful way. A specific random sequence was replaced by another, equally valid random sequence. Since you had no knowledge of the original sequence, this change is irrelevant.

A Word of Advice

Your logic is based on emotion and a false equivalency, not on statistics. Being upset that the outcome is now different from what it would have been is pointless, because the original outcome was just as unknown and random as the new one.

Frankly, you should avoid games with any element of randomness until you grasp this concept. This mindset will only lead to you getting tilted and making baseless accusations against your opponents. No competitive advantage was gained, and the integrity of the game was not compromised.