What is the ruling on a man mixing with his paternal aunt through rada'a (his father's sister through rada'a) and exchanging praise and gifts, and what is the extent of validity of his claim that she is like a mother to him, given evidence of her beautifying herself for him?

You described your husband as religious, but his piety should keep him away from temptations, not make him seek pretexts for them. Satan strives to tempt people, and Iblis sends forth his legions to sow discord between a man and his wife. Mingling with women is forbidden, and there is a warning against their temptation. The sister of your husband's father's wife is a non-mahram to him, and her age does not justify leniency in dealing with her, especially if she adorns herself in his presence. Advise your husband to fear Allah and be sincere in his intentions. Continue to advise him with kindness and pray for him.