Totally silly. The g/f wasn't even a fixture in your son's life, as you clearly point out, and they were not dedicating themselves to the normal route of marriage > house > children and a settled life.
For you to get emotional about something no-one knew about, least of all the prospective mother, is foolish. According to you, it happened months ago.
This g/f might only have been a passing fancy for a bit of a holiday adventure and they may have parted in the not too distant future, therefore, for you to get wound up about something which would have put the brakes on their plans even more abruptly, is plain silly.
Would your son have stuck by her with twins on the way? They both have failed relationships and children to support and if finances are 'a burden' now, with two on the way, would the relationship have lasted. Think about it. The whole scenario is not one you'd want to bring a child into as they aren't even in a comnitted relationship.