As for the girlfriend issue, the less you wait the better. like this she is staying with him and bonding to him more and more, and the more you wait the harder it will be on everybody. plus, if they're having sex too then you know, there's a chance that they also end up like you and him. NOT a good scenario, right?
don't expect not to hurt her nor expect her to be understanding or to stay your friend and his girlfriend after all. anyone would be hurt and pissed in her situation.
if I were you i would face her in person, if possible both of you together. at some neutral place where she or you both can leave anytime. if you can't do it like that, then i would write a letter together with the FOB and give it to her. texts, phone calls, whatsapp and especially RUMORS... are the worst and defo a no go. don't forget to apologize and don't try to make excuses and justify your act.
good luck with that. it is one of those uncomfortable "ugly" things you need to go through in this journey... but the sooner you get it over with, the better. when your baby comes it will all become much less relevant and much less of a torture than it is now. with this, i don't mean to say that sleeping with your best friend's boyfriend and getting pregnant was a smart thing to do, but as you said yourself, you live and you learn, also from not such smart moves and mistakes, right?