Like mmm said, user_register() is not for displaying additional content, but for hooking into the internal user creation flow.
What you will need to do is to hook into user_register() to store a short-lived transient marking that the user has registered. Then, add your modal code to the wp_footer() output, wrapped in a test for the transient you just set to see if a new user has been created (not forgetting to delete the transient immediately afterwards).
Hope that helps
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