When an unconfirmed transaction gets dropped from the mempool, all transactions that used the unconfirmed transaction as input will be removed also. This means your friend's transaction (and any transactions that used it as input) will be dropped along with your transaction

If you would prefer the transaction to be confirmed instead no matter how long it will take, you can push the transaction back to the mempool every time it gets dropped. To push a transaction, simply go to the sites listed here and enter your raw transaction. You can get the raw transaction from bc.i by adding "?format=hex" at the end of the transaction link like this. Make sure you save it (and your friend's raw tx) because it won't be available once bc.i drops the tx