Do you advise me to isolate myself for memorization for an exam, or to mix with a weak-in-faith colleague?

If it is possible for you to avoid associating with someone whose harm you fear, that is better; because the Shari'ah encourages the companionship of the good and warns against the companionship of the evil. However, if you are in need of him, there is no harm in associating with him if you predominantly believe that you are safe from his harm and will not be influenced by him, while enjoining good upon him and forbidding evil from him with wisdom. If he does not desist, then part ways with him. It is incumbent upon a Muslim to keep in mind the matter of inviting to Allah and guiding those with whom he associates.