Humidity is the amount of water vapor in the air. High humidity can cause corrosion, condensation, and short circuits in electronic devices. Low humidity can cause static electricity and damage sensitive components. The optimal humidity range for a data center is between 40% and 60%. Based on the report, the humidity level in the server room is 70%, which is too high and can affect the performance and reliability of the network equipment. Therefore, the best recommendation to the client is to install a humidity sensor and a dehumidifier to control the humidity level in the server room.
References: Network+ Study Guide Objective 5.1: Summarize the importance of physical security controls.

1. Identify the problem. Done
2. Establish a theory of probable cause. Done
3. Test the theory to determine the cause. The question states that he has "determined" the cause. Done
4. Establish a plan of action to resolve the problem and implement the solution
5. Verify full system functionality, and, if applicable, implement preventive measures
6. Document findings, actions and outcomes

A role-based access control (RBAC) policy is a security measure that assigns permissions and privileges to users based on their roles and responsibilities within an organization. RBAC helps to enforce the principle of least privilege, which states that users should only have the minimum level of access required to perform their tasks. RBAC also helps to prevent unauthorized access, modification, or misuse of sensitive data or resources by limiting the scope and impact of user actions.
A software patching policy, firewalls on the software development servers, and longer and more complex password requirements are all good security practices, but they do not directly address the issue of preventing the former software developer from modifying code. A software patching policy ensures that software is updated regularly to fix bugs and vulnerabilities, but it does not prevent a user from introducing new code changes. Firewalls on the software development servers protect the servers from external attacks, but they do not prevent a user from accessing the servers internally. Longer and more complex password requirements make it harder for attackers to guess or crack passwords, but they do not prevent a user from using their own valid credentials.
Reference
1: Role-Based Access Control (RBAC) - Definition and Examples
2: Network+ (Plus) Certification | CompTIA IT Certifications
3: [What is the Principle of Least Privilege? - Definition from Techopedia]