Discuss and learn French: French vocabulary, French grammar, French culture etc.
I will guess that it means that she went out of her way to help me (literally she put herself into expenses in order to help me).
Since "frais" means "expenses " that is my guess.
I think the opposite might be to do someone a favour when it is no skin off your nose.
That's correct Georges. Se mettre en frais is To do something at one's own expense