What is the statistical possibility of a girl getting pregnant?

Before you guys freak out, no, I did not bang anybody up. This is purely a statistical question, and I need it for a D&D game I’m playing. One of the players today had sex with an NPC. She is a human sorceress (healthy 25 yo) and the NPC was a 50-y-old nobleman (also healthy). Regardless of the storyline, I wanted to know what her chances are of her getting pregnant, so as to include it in the game.

So, assuming neither person is sterile (or we could also add that into the equation), and notwithstanding the possibility that she may have had her period (something I need to take into account in the future it seems) what is the statistical chance of someone getting pregnant when having a one-night-stand?

for those unfamiliar with D&D rules, there is a combination of 2 dice that allows you to roll numbers between 1 and 100, so if I know the chance is 13% for example, I’ll have her roll the dice, and if the number is between 1 and 13 she’ll be pregnant, if higher, she won’t