First, I"m not sure what sort of LEGAL advice you were seeking in regards to your husband adding his ex as a friend on Facebook. He can add whoever he wants and it really is NUNYA! If you don't like it, whine to HIM about it.
Second, if someone tells you to do something utterly stupid and inappropriate, there's this nifty little word....NO! As in "No, honey, I'm sure your ex is feeling nervous and stressed and in pain, and it really would not be appropriate for ME, the new bed buddy to be there. So you run along and enjoy your child and I'll share your happiness with you when you get back." But wait....that would take a TINY bit of class, and that's something you and your hubby obviously lack. Dang, no woman wants
Third, just remember when he dumps you and moves on to number 3 (or 5 or 10 or whatever number she's going to be) he's going to tell HER that he's "got the woman he wants". Oh, and he won't support YOUR child either and you'll be here wanting to know if you can interfere with his visitation or how to terminate his rights so that your great new man can adopt your kid.