I have a friend whose husband 'lacks' gheerah (makes her freemix with his brothers/meeting his colleagues/going outside with incorrect hijab/make-up, etc.), thereby creating fitan for both of them, without them realizing how harmful it is. How can I advise her without creating issues between them?

If you think it is wise to advise, then advise her alone and gently, and give her the ruling rather than a criticism of her husband: her mixing with his brothers is not permitted, and the Prophet ﷺ said about the husband's relatives, the in law is death (al-Bukhari). If she does not take it from you, have someone she listens to advise her, and a man he listens to should advise him.