As a Scrum Master, you are accountable for ensuring that your team has a productive Sprint Retrospective at the end of every Sprint. Often, this means facilitating the retrospective. However, sometimes, it may also mean finding someone from outside the team (who is accepted by all of the members of the team) to facilitate so you can be an active participant in the retrospective.
In the case of a new developer on the team, has anyone else raised this as a concern? Teams can raise concerns and impediments at any point in time - they don't need to wait for the Sprint Retrospective. Someone raising a concern - either as a team at the Sprint Retrospective or between individuals in a more private setting - is a good first start. Open and honest discussion and agreement that at least one person perceives a problem is a good start. Figuring out solutions is the next step.
As a Scrum Master, you may also observe impediments or problems, but it's important to make sure that the team remains self-organizing and self-managing. It's very easy to slip into a directing or managing stance if you are pointing out problems and/or leading the team to solutions too much.
There's also an aspect from outside the Scrum framework - management. The management of individuals - things like hiring, onboarding, performance management, and offboarding - fall outside of the Scrum framework. If the team is raising concerns about the abilities of a person to contribute to the team, one path forward would be through the person's manager to see what options exist for setting expectations and making sure that the individual is set up for success on the team. And if the person does not make progress toward expectations, it may become necessary to remove them from the team and organization.
