Without being in a position of team lead, the options are somewhat limited. There are different bases of power. The individual in your question won't have legitimate power, as they are not the lead or manager and they don't have the position to drive change. However, depending on their personality, they may be able to develop a position of referent power or expert power, but that will take time. Using coercive power likely won't be helpful in enacting changes.
If there is management support for implementing these good practices in the team, then having someone familiar with the practices, the team, and the technical components of the project would be extremely valuable. However, they would need to work closely with the manager to enact any changes. The individual would need to be in it for the long-haul, and understand their role in facilitating management's vision.
If neither management nor the senior members of the team want to change things, though, I think it's very unlikely that anything will change. Change needs to come from somewhere, either top-down from management or bottom-up from the team. If neither side is willing to support change, then change won't happen.
I would be concerned with the ability of the developer to leave. Working in the conditions that you describe - other developers who do not continually improve or mature their skills, lack of good software engineering practices, and not being in a position to drive changes to make the working conditions better - could lead the developer to leave the team or the organization to find a better fit. If the developer continues to work through it, they could see higher levels of stress, anxiety, and even burnout.
Alternatively, driving changes top-down could alienate members of the team. If you have people with very specific, business critical knowledge and you made changes that they see as difficult, they could become disgruntled and leave the organization as well. Management needs to be careful when rolling out changes to begin a transformative process, but not disrupt individuals or business activities.