Someone who is "devout" is nonetheless capable of sin. We are weak creatures; it is one thing to say you will do the right thing and another to actually do it if and when it is required, especially if the consequences mean social isolation or rejection by family.
Sadly, when such a situation arises, many people give in to their prejudices, fears, or pride and ignore what they learned in church. She should be asking for forgiveness for what she did to her child, not trying to justify her actions.
No, I find it hard to believe that "social isolation" or "rejection" would be major issues for devout Catholics. Devout Catholics are active in their parishes, and hence, find most of their social and peer groups there. Even in the early 70's there would be little sigma for the family of a biracial baby in a Catholic peer group.