Should I tell the pregnant girlfriend?

[me]

Forget teaching him anything and instead focus on what the right thing to do is. If your intentions are vengeful things rarely turn out well. But if you feel that she is being betrayed and deserves to know, those are valid reasons to tell her.

I would send her a brief message explaining what happened and let her know she can ask you any questions. But if she doesn't take it well, blames you, etc. then let it go and let her deal with it herself

Edit: to mimi - She is not the one ruining anyone's life, the father is by getting women pregnant then cheating on them. Why blame the person who brings the bullsh*t to light rather than the one causing the BS in the first place?