What's the problem? It's not like you didn't know your mom fucks, or that she's a fine piece of ass. Was it paid porn, or amateur porn? If it was paid porn, then she did it knowing full well you might one day find it, and jerk off to her, and did it anyway. If it's amateur, then there's every chance some guy fucked your mom's brains out either without telling her he was filming her, or promised to delete it after, then uploaded it to humiliate her.
So it all depends how n what reaction you want. Do you want to shame her? Convince her it was OK, so she considers doing more? Let your mom know her little boy watched her get fucked, and plant that seed of doubt of "did he like it?" in her mind?