Tell the girl yourself... Express your condolences and then explain to her that she doesn't need to be texting him all the time.
And tell SO that whether or not he thinks that you're making it bigger than it is, he can still respect how you feel. As long as he's responding to her, she'll keep on. It's yours and his responsibility to protect your relationhip... Not hers. Why should she care?
after the bewbess of this dies down if tgis doesnt stop id calmly just say "I understand you're trying to help her through this but she's vulnerable and I think this behavior is leading her into thinking there's more there than there really is. She's very vulnerable and lonely, she can't think straight and is probably trying to replace that romance. She can't be replacing it with you. If she's trying to fill an intimate need it can't be with a married man. You can be there for her to an extent but you can't get that involved. Being there to have her baby is a job for her mother, not you. That's an Intimate moment between a man and woman and I'd like to be the only woman who experiences that kind if Intimacy with you."