Depending on the situation sometimes no one is there at all. Most boyfriends find it shocking & kind of hard to grasp. But then there are a couple who are really supportive like many husbands out there
But if she is living 'in" with the boyfriend then yes he will able to do what a legal husband does. The term legal husband sounds kind of funny. Isn't the word husband legal enough.
It also comes down to how ready the couple or the father involved was for a baby. Not all husbands prove to be supportive either.