I am borrowing my friend's pokemon leaf green. Yes I know the version but I don't want to delete his data. Is there any way so when I use the gameshark, I don't need to worry about the data?

As long as YOU DO NOT start a new game or SAVE it then your safe!

As for backing it up: Currently it can not be done since Most or ALL the pokemon games does not allow the MadCatz Game Shark SP and PC App (Sharkport) to upload a game save to the cart. You can back it up is some cases but you WILL NEVER be able to put it back!

But what about all the other pokemon games that I used gameshark on? Such as Mystery dungeon red rescue team? ( red rescue team is my game). In red rescue team, I saved but nothing went wrong. Is it only in leaf green that the data erases?

The Game Shark SP DOES NOT ERASE data! It only keeps it if you saved it!

Like I said before your safe as long as you don't save!

If your worried about the game becoming corrupted then don't save it! If you want to backup the data then you can but like I said the GAME prevents it from being loaded back on, this applies TO MOST pokemon GBA Games! LG and FR are one of them. It will also not work on other games as well. It has something to do with the system clock!