Dynamically assign an IAM Role between an IAM user and and EC2 instance

In short: Please don't!
Instances (systems) usually have a usage pattern that is totally different from humans. Mixing up their rights is a bound to conflict with the principle of "least privilege".

If somehow you have a use-case that requires this (please double-check!):

• Write the policy into a managed policy
• Attach it to the instance profile
• Attach it to the user (or better, their group)

Update the Trust Policy of the role to something like this:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "Service": "ec2.amazonaws.com",
                "AWS": "arn:aws:iam::0123456789012:user/johnsmith"
            },
            "Action": "sts:AssumeRole"
        }
    ]
}