That's right, the mempool now are very congested that cause many pending transactions, most commonly on this case, it takes 7 days before it will reject by the network your transaction.
What happens to his transaction after? It will simply burn fees and get back to his wallet (automatically?) or or his transaction will stuck someone in blockchain and he will have to spend extra funds to move or accelerate it ?
Ain't experience this but AFAIK, if the transaction was rejected by the network it is eventually disappears from the network and back to the sender's wallet after a few days of patiently waiting.
So, while there's alternative way to do why you will wait for how many days if you can afford to increase the fees by using the First-Seen-Safe, Replace By Fee(FSS-RBF). Here is a
Youtube tutorial to make this things clear for the OP. Too old but seems have useful content.