Pokemon cant forget hm thay can only forget TM
fly is an HM and no Pokemon can forget an HM.
Pokemon cannot forget HM's unless they're in Pokemon Mystery Dungeon. But, if its from Emerald, add another Pokemon to your party that knows surf and ask the move deleter.
There is a move deleter who can make pokemon forget HM moves.
take the Pokemon to a guy in lilycove city and he makes them forget
you cant
You can just reteach it since HM are reusable.
no their unforgetable
Find the move deleter.
To delete an HM move, you must go to the move deleter in Fushia city. He can delete any move you want to... even a HM move! but don't forget that after that HM is deleted that Pokemon only has 3 moves left so teach it another move!
talk to the move deleter
move deleter in lilycve (end of game)
Go to the Move Deleter in Canalaeve City. He will make your Pokemon forget any moves and is the only way to forget HM moves. If you want a move back (not including TMs/Hms) go to the Move Relearner in Pastoria City with a Heart Scale, which can be found underground.