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 ?
Someone correct me if I'm wrong, but you get the entire amount sent back to the originating wallet. Makes sense, because the transaction was never included in a block and so the miners shouldn't get any fee.
Basically yes, but better said the amount is not sent back as the coins never left, it's just the pending transaction is dropped as it never happened.
Is it worth it will it work ?
It will work alright but if it's worth it or not that's for you to decide, for that fee, you would have paid 120sat/b and get a confirmation in probably an hour a two in the first place.
You could at least try and wait a bit to see if the pool will not clear again as it usually does early in the morning (GMT time), that's in 8-10 hours from now if that's not urgent and make a decision then.
I wouldn't pay close to 10% for a tx unless is something really essential.
LE:
You realize that to pay the fee you also have to deposit BTC in your viabtc account, so better think of paying that in some altcoin as you will end up paying double.