He can do it multiple times, and it's not a tap effect, nor a mana requirement
If you get something powerful, you might even want to keep its original owner around just to maintain access to it
Later game you can play Silverquil while holding up an instant and use Silverquill as casualty fodder in response to removal
Number 53 Language English Not Legal Formats Alchemy, Brawl, Commander, Competitive Brawl, Historic, Legacy, Modern, Pauper, Pioneer, Standard, Timeless, Vintage Rulings (2/29/2020) If Enter the Dungeon is cast during a subgame, a sub-subgame begins, preferably under a new, smaller table, but do what you can
That's what we're looking for here - the cards that are already starting to move up and were valued lower on day one than they are today