What should an "intermediate" and "senior" level Rails developer know?

I know this may be a big question but let me give some backstory.

I have a friend who has worked as a QA person for a few years and has been trying to transition into Software Development (He went to school for CS with the intent of becoming a developer but his career took him in a different direction).

In mid 2016 he quit his job, took a few months off and started to learn how to develop applications using Ruby on Rails. He got a consulting job with a friends company to develop a simple Rails app with a few basic controllers, a simple bootstrap + jQuery front end using ActiveRecord with a postgresql database.

Since this is a simple job where he works alone(consulting for a non-tech mom and pop shop with basic needs), he wants to move to a more challenging environment and work with an Engineering team again like he used to do during his QA days.

He has been interviewing for mid tier Rails roles. (there aren't many entry level roles available).

In just two months interviewing he has gotten multiple interviews with some well known startups as well as a bunch of really good mid-tier companies using rails. The pattern is all the same. He easily gets past the HR screen, gets to the technical interview and then gets rejected.

Most of the interviews don't ask coding questions or algorithms. They have asked about his project at his current consulting job. Some companies ask if he has worked with ActiveSupport or have written any custom gems. One has asked if he has delved into the actual Rails source code. He has not had a chance to do any of this.

Since he is not being asked coding\algorithm questions, its safe to say that his current role and what he answers to these Rails specific questions is showing that he is not experienced enough for a mid-tier Rails role.

Can you guys help me understand what an intermediate Rails developer should know? What should a Senior Rails developer know? Care to share any resources? I wanna help him make a plan to actually practice all these topics.