Bestfriend not being a good mother please help?

I really need someone to talk to,becuase I am so stressed out,and I dont know what to do anymore.My best friend has been wither boyfriend for years,she just had his baby about three weeks ago,About two months ago the guy i have been inlove with for three years,moved in with,me becuase he got kicked out,he stayed with me for a month,then he found his own place, about a street away from sarah,my bestfriend.him and sarah have beengood freinds for six years,now that he lives so close he is at her house constantly, its quite clear that she likes him,he callsher wifey,and that hes thefatherof her baby,they say theyare just joking around.when she needs to go shopping for the baby,she invites him,and not her boyfriend,she broke up with her boyfriend yesterday,for the past week,he has been calling me,crying,he loves her with all his heart,and would do anything for her,and this is breaking his heart,i have been crying becuase the things he tells me,is also breaking my heart,she told me yestreday that she doesnt like him becuase hes ugly,and hse doesnt want to spend the rest of her life with him,she said she wishess she never had the baby,she misses he rold life,were all twenty by the way,i have told her,im so dissapointed in her,her family should be her top priority,not the guy who moved down the street.I have thought about talking to him telling him to back off and let sarah and her babys father spend some time alone together,but he will just get mad at me and say im starting trouble.but they are together constantly.I know this isnt really my business but all three of these people i care very much about,and this is stressing me out,the way shes acting,does anyonehave any ideas of what i can do about this?thankyou so much