As i understood, your friend has hexed someone and it backfire. Now he wants to remove the hex, right?
Any spell cast has the possibility to bring unexpected and detrimental results. For that reason one should always know how to reverse any spell cast.
He will need some angelica herb, rosemary oil, a piece of chalk or colored duct tape, a lighter, five white candles, a foot of string, benzoin incense, an incense burner, a compass, a piece of black cloth, a fire-proof plate or container, something of value to you. Typically the item of value is best if its jewelry, even the fake kind. It just needs to be something you like or have possessed for a long time. Be warned. He wont be getting this back.
He should start the ritual at the same time of day and in the same spot where the original spell was cast if at all possible.he has to Face east and use the chalk to draw a complete circle around himself. If he cant do this because of the surface he is on, he might want to use the colored duct tape to lay down a circle. From this point forward do not cross the circle for any reason as it will break the ritual and you will have to start over. He has to Put the candles out near the rim around himself in a circle.He has to Anoint the wicks and the candle signs with just a few drops of rosemary oil, then light them. He has to Put the benzoin incense in an incense burner and light it.
He can chant anything, her's and example: I cast a spell asking (Insert type of spell here); I now ask the favor of having the spell removed. I understand to take back a spell means giving up something of my own to show my spirit is true and my intentions are good, I give this (Possessions name). I transfer the spell into the (Possessions name) and render the spell broken. No harm may come from the ending of this spell. No further power shall it have. This is my will. So shall it be."
Then he has to put the item and the angelica in the center of the black cloth. He has to lift up the four ends of the cloth to make a bag.Tell him to tie the end of the bag shut with the string. He ought to place the bag on the fire-proof plate; pour wax from one of the candles onto the top of the bag and the knot he tied. Tell him to light the bag and let it burn down to ash. If theres anything left that the fire wasnt capable of burning, he must throw it away as soon as you are able.
Then he will break the circle and clean up your ritual supplies. The spell he cast should be undone.
This is my suggestion for a reverse spell, of course he can look into other things.
This is pretty complicated to do something with the backfiring thing, he should have thought about the results of the hex before he have done it without having any experience.
If he is over it and he has now good motives, the back firing thing might be solved out, but this already harmed alot of people- i do not promise immediate results!