Last time I sent some bitcoin to my friend and the fee that I used was a little low, so I tried to broadcast the tx with online broadcasters. After that the tx was spent from my friend's address (not from mine). It showed that my tx confirmed, but as it confirmed another tx was initiated from my friend's address without the permission of him and was spent to another address what kind of attack is it? I think it's not called double spend since it's not spent from my address, it reached his address first but was then spent from him. Can someone explain??
If I read it correctly, your friend had an address, to which you paid. Then someone took it from their wallet?
It might be because your friend's device is hacked or he doesn't use a trustworthy wallet.
Or it was him who spent the coins. It's also possible that they are lying to you so that you pay them again.
Double spend attacks involve the same person spending to two different addresses. Since your friend is not the voluntary author of both transactions, this is not double spend.
I can't comment further without more info.