This is a tough one. No, you don't have to be a part of it. You can choose to opt out. But maybe pass along what you have first. Or at least to the man's "legitimate" children, if any (they might want to know they have half-siblings out there). Or other relatives who might want to be in contact. Good luck and I hope you can work something out that helps them find out about their family but leaves you and your mother in peace.