I can see where you came from... There are 10 possible rolls he could have done on the first attempt and 10 possible rolls he could have done on the second attempt. And 1/10 + 1/10 = 1/100
However, there is a total of 6 ways to reach 13 (since there is a 0 instead of 10)... 4+9, 5+8, 6+7, 7+6, 8+5 & 9+4, which makes it 6/100 or just under 17/1.
Errr no. It's just a tenth. He had already seen that he had rolled an 8 before he posted his second comment, leaving 1 success out of the 10 possibilities.
If he didn't look at his first comment before posting his second, then you are correct, but the chances are he did.