Un noticed illegal move

What happens if a player(A) makes an illegal move

But the opponent(B) does not notice it andplays his move.

After that can he still appeal for illegal move

Depends on when it is noticed and what time control you're playing with. It also depends on what regulations are being used.

It's going to be hard once B has completed his move.

Classical time control:
7.5.1     
An illegal move is completed once the player has pressed his clock. If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. Articles 4.3 and 4.7 apply to the move replacing the illegal move. The game shall then continue from this reinstated position.

Rapid time control:
A.4.2
If the arbiter observes an action taken under Article 7.5.1, 7.5.2, 7.5.3 or 7.5.4, he shall act according to Article 7.5.5, provided the opponent has not made his next move. If the arbiter does not intervene, the opponent is entitled to claim, provided the opponent has not made his next move. If the opponent does not claim and the arbiter does not intervene, the illegal move shall stand and the game shall continue. Once the opponent has made his next move, an illegal move cannot be corrected unless this is agreed by the players without intervention of the arbiter.

https://handbook.fide.com/chapter/E012018 

In the absence of an arbiter, this is going to be difficult.

If one person makes the 1illegal move but the other person does not recognise it that it was an illegal move and tells it afterwards that it was a illegal move and he says on its turn so does the player who did the illegal move get turn back and play immediately when he is not getting a a check or he must take his move back and then play the next move where is not getting a check