There's a long old road between a baby being born and them being adopted, so the first thing to think is not to rush. Even if the birth mother would not care for the baby, and there were no family members, it would still go I to foster care whilst all options were explored. I believe that there is a 6 week waiting period for any adoption procedures to begin after birth regardless.
I think whether you should give advice depends very much what your relationship with your friend and her daughter is. Her daughter will be getting advice from many sources, including her mother, and at the very least a social worker, midwife and health visitor whilst any decisions are made. They will be supporting her to keep the baby in family (including birth fathers family) if at all possible and helping her to make informed decisions.
The problem is that there are lots of factors at play, and whilst something seems right (I'm the first to advocate for children remaining with birth families if at all possible, the trauma adoption causes should be a last resort), I would be questioning why I felt so strongly that the daughters wishes should be overridden, and more importantly, what I could add to the situation beyond a general vibe that children should stay with their birth families (which they will be hearing from society, all professionals and every Tom Dick and Harry who insists on telling them that in the Tesco grocery line. Unless you are able to give specific advice or support (i.e. you have professional or personal experience), or are willing to step up and make a very concrete, out of the ordinary and extreme commitment that she wouldn't expect you to undertake, but may be useful were she to adopt the child (for example offering an unconditional night a week childcare for the first 5 years of the child's life, financial or practical support in the medium to long term or an offer of accomodation to a party involved) i would stick to what you are good at and have experience of, namely being her friend. She hasn't told you herself yet, which makes me suspect she probably needs to be able to pretend to someone that her life is normal and she isn't going through this. She might appreciate it if you could all just play dumb for a bit so she can escape from it when she speaks to you. She probably needs friends right now, and wants them to do friend things, like take her out for a glass of wine and listen to her bitch and moan, or deliver a tonne of chocolate to her door, or even just gossip about the latest season of Bridgerton to take her mind off it unless she asks you for your advice.
She is probably navigating a whole host of tricky emotions- being scared because her daughter is in this situation, scared of becoming a carer for the baby, scared of the judgement of everybody whatever her daughter decides, delighted at a new addition whilst being terrified of it being taken away from her without her choice, or being forced to choose between her daughter and grandchild.... It's all so emotional and so difficult, and probably affected by all sorts of hormones. Living with a 16 year old is tricky, or so I have heard, and I can imagine that pregnancy and post partum will do little to make that a less tricky situation! She might be worried because of all the social workers this will inevitably bring into the house, and although you say it is good conditions, I can imagine that any parent who found their 16 year old was pregnant might feel worried If a social worker came round and started to ask difficult questions. There is a possibility that your friend might be assessed and found that she isn't suitable to adopt the child, even if she wanted to. She might be worrying about that, too. If emotions are running high, I wouldn't expect even a highly logical and thoughtful opinion to be welcome, despite how strongly you may feel, and if she feels sensitive or cagey, any attempts to give your opinion might be seen as just another person judging them. There will probably be little to say to her that she hasn't already thought about.
You do sound like you are really concerned about her, so I would focus as much as you can on making sure you can support her as a person before giving any unsolicited advice that might well be misinterpreted or fall on deaf ears at such a tricky time. Having someone there who is completely non judgemental and ready to just listen without proffering an opinion or solutions would be invaluable at this time I would imagine!
If you are super keen and desperate to offer some support, you could consider gently trying to work out why the daughter feels unable to keep the child, and where you could direct her to find support. She may feel no bond, it may have been concieved in difficult circumstances, it may interfere with her career plans, it may not be financially viable for her, she might feel frightened about birth father finding out, she might be scared of judgement, she might worry that she will be be isolated from peers.... All of those are valid reasons, which can be overcome in one way or another but require her to want to do so and probably a lot of hard work. You could offer to support her by writing applications for benefits, researching food banks, offer to accompany her to medical appointments If the relationship between her and the mother has broken down, research charities that offer support with baby gear, research her eligibility for childcare options, help her to write applications for jobs that will support part time working etc. If the child is taken into care, there are agencies like Pause who can support her afterwards.
There are quite a few other threads on here from people who have considered adoption for their children, and it may be worth gently suggesting to your friends daughter to see some of those or post on here herself if she feels able. Although the adoption board is obviously adopter heavy, from my experience people have given solid and compassionate advice to people who have been considering this.
Generally, with delicate situations like this, I would say I less you have been specifically asked to contribute, just let her know you are there for her no matter what and let her take the lead on what to do next. Give her the gracious option of being able to pretend to you that's everything is ok, whilst being prepared to offer the support she needs