Who is right...

So this room came out on my sons turn, he had sacred heart, i had dad's lost coin. He ended up with at least 10 treasures because he kept changing his 1 to a 6. But since I had dad's lost coin, should I have been able to counter his 1 for the reroll or was he right in changing it straight to a 6?

So this room came out on my sons turn, he had sacred heart, i had dad's lost coin. He ended up with at least 10 treasures because he kept changing his 1 to a 6. But since I had dad's lost coin, should I have been able to counter his 1 for the reroll or was he right in changing it straight to a 6?