Heal, welcome to A2K, even if the circumstances aren't the best. I hope when all this is done, you will stay around.
You need to give us a little more here if you want optimum free advice. Why did the mother wait over a year to inform your boyfriend of the situation? Exactly what is she looking for from your boyfriend? Was she going to handle it on her own but now needs financial assistance? If this event happened at a work party, how did it go so long without him finding out? Did he not add up two and two when Mary in payroll started showing? Are you sure this news is as new to him as it is to you?
Regardless, that he has another biological child in the world does not mean that something has been denied to you. The other woman bore a child, but she did not share a pregnancy with your boyfriend. He did not feel the baby move or kick, he did not share the miracle of birth or the joys and hardships that come afterward. You two still have all of that to share together. Other than a possible financial impact this doesn't seem to be the disaster you portray it as unless there is something to the story you haven't shared.