Birth Family are not your friends, and they do not have your childs best interests at heart - they will have a rose tinted view of their brief time with your child, and will want to make the case that whatever happened was not their fault. Dont feel sorry for them, they either abused or neglected your child, or posed an imminent threat through drug or alcohol use - but they will present as being perfectly reasonable because they are well rehearsed in 'working the system' - until that is they dont get what they want and then the gloves will come off.
They do and will pose a threat to your child and the unity of your family, and at some point in the future (thanks to social media will either initiate contact with your child or will respond to your childs curiosity in them). They will not behave reasonably (in any way that you will count as reasonable any way) - your family values will not be compatible in any way with theirs and they will expect to co-parent your child once you have finished baby-sitting for 16 years.
They will probably bide their time, making contact with your child at 16 or just before because they will already know that the authorities will only act to protect a minor. Police will not intervene, even though they will be effectively grooming your child, and social services are both powerless and ineffective.
When they do make contact, they will love-bomb your child using the combined force of their (probably very large family) from aunties, cousins, and siblings throughout the day and late at night (because they probably dont work / attend school) with a flood of sm messages at your childs lowest point (perhaps when you have fallen out, or they have fallen out with their friends, and when your child is seeking validation from somewhere else) this will confuse your child, reignite painful memories, or manufacture false memories that paint them in the best possible light as fellow victims of SWs and the police.
Adoption is not financially rewarded, and if your family will be considered an adoption success by social services, the adoption service where your child came from will not help or advise (because they are overworked and do not want to count yours as a family in crisis when they have already counted it as a success). Your local authority adoption service will not assist because they dont want to add your case as a failure when they didnt play a part in the original placement and spoil their stats either.
Just work on your bond with your child, dont let bf become some romanticised mystery, respond only when your child shows interest, and respond only with facts, constantly reinforce that they let your child down and that even though it's sad that it happened it means that you are all together. If you get SWs involved that will only devalue you as parents, if you promote bf, then you further dilute yourselves as parents. 'normal' children have no concept of any this - perhaps the closest analogy is where parents have split up and found new partners and each is seeking to undermine the other.
Adoption becomes more and more complicated, so build lots of memories while your child is young and give them experiences that make them completely incompatible with their bf, because they (bf) will present themselves in a very alluring light with sm posts on facebook, instagram etc that you (no doubt) have been advised to stay clear of for your childs safety.
Good luck