I think that pretty much anyone you talk to will have had some sort of experience with people like this in the workplace. The important thing I think is to be able to distinguish between a few key characteristics in the unmotivated, mal-performing coworker.

1. They may be trying really hard on their own and just can't get anywhere at all with it. Being in an environment with so many brilliant coworkers, it can be extremely uncomfortable to let anyone know that you're struggling with something. Having an experienced and somewhat idyllic senior engineer volunteer their time to help can be a big step in opening that person up to a whole world of knowledge that seemed unreachable before.

2. They may not be interested in the particular project that they have been assigned. Although it's not much of an excuse for not working, a lack of motivation and excitement in your job can really give a bad impression to everyone around you. Perhaps the person simply needs to be redirected to something they can throw themselves into and really make a difference.

3. They may just not give 2 shits. At that point, there's nothing left but either confront the person (especially if you're relying on their performance in any way) or bring it to the attention of the supervisor.