It is unlikely unless his penis got by her vagina.
Pretty much 0, sperm has to have a way to reach the egg. It cannot swim through the air so while it's theoretically possible for conception to occur if for some reason semen got inside the vagina without penetration -it's just so incredibly unlikely that it would be enough to get pregnant. Also a women cannot get pregnant if she isn't ovulating or isn't about to -it's difficult to know exactly when ovulation occurs and sperm can live for up to a week inside the vagina so it's not safe to assume you can't get pregnant (because this assumption is one of the highest reasons for getting pregnant). But the point is just because there is semen in the room doesn't mean a women can get pregnant; semen needs to find a way inside and up to where an egg will be and an egg needs to appear sometime within a week.