It seems like this is getting a bit heated and is based on speculation about a family that is clearly having current and active involvement with SS. With such limited information it is difficult to make any realistic, kind or appropriate comments, and I would encourage everyone to bear in mind that if we don't know what is happening from the original person, it's not fair to speculate.
However, OP, if you are wanting genuine possible reasons, there could be many reasons that a younger child might be placed for adoption whilst leaving older children in care.
It may be that there are better support networks for older children (family, school, mentors etc) that aren't there for the younger children, or that the older children have a father figure or family support that are able to act based safety factors. Maybe younger one has a different dad who has not been assessed as appropriate either, and whose family might not want to pursue guardianship.
Its possible that their mother has shown she has capacity and support to care for the older children but less so for the younger child. It's lso worth thinking that children don't get any younger, and without appropriate and sustained change, the younger child will have the same environment as the older ones, so it's not about a baby being easier to manage necessarily, it's about whether she is able to change whatever factors have led to them being in care quickly and sustained enough to mean she can care for the baby and prevent the same thing happening to them. This can be made trickier with older siblings in the mix, especially if the older siblings are drafted in to help care for the parent or other siblings. It can entrench unhealthy dynamics where a child is parentified.
In terms of keeping them together, it may be that the siblings have been placed apart (often the older siblings will be placed together and the younger one in a separate placement as 3 child foster placements can be hard to find) so it may be considered that separating the older ones would sever a deep sibling bond, whereas for the younger one of they have been in a separate placement for most of their life, it might be less of a strong tie than the older ones.
There is also the harsh truth that younger children are easier to adopt, so if she can only cope with a certain number of children, and that situation is not likely to improve in a meaningful timeframe, it is possibly more likely the younger one will be considered for adoption as it is more like to be a successful process. It would be unlikely (not impossible) that a child over 8 would be placed for adoption as the likelihood is they would be nearly 9 by the point of adoption. Children over 5 are considered harder to place.
All that being said, all of that is complete speculation, and I would caution that unless you are privy to all the information (e.g have been in the meetings etc), the best support you can give to your friend is getting her the support she needs to go through it. I can highly recommend the organisation Pause, which helps mothers who have had their children taken into state care, or getting her to speak to the social workers to get some counselling and family support.
I can assure you, adoption is the least favoured option for any child, and I would hope that you could use anything learned on the forum to give kind and supportive advice to anyone involved if they require it. Adoption is not thrown out lightly and I would caution that phrases like "adopted out" make it seem that you consider adoption more like a temporary care situation, rather than a permanent, devastating ending of the legal relationship between parent and child. I'm sure it was just clumsy phrasing, but I hope that you can see why it might be tricky on an adopters board for that wording to go unchallenged.
I say all of this as an adopter that desperately wishes her children's birth mother had some friends who were able to support her to access the help she needed in a genuine way. If there is a way for you to help this family get them help they need, please use any experiences or advice you have to get them to do it. Sometimes just knowing that someone else has a working knowledge of the social work system or can speak to people who do can make the difference between them being uncooperative and them engaging with those very services that are trying to support them.