Usually used to describe friends who you have caught out lying, or doing things behind other peoples backs. People who are a bit sly, misleading, calculating & sometimes will lie to make friends!!
Generally, these people are not bad people, there's just something about them that you know is not quite right! Their actions show a side that makes them come across as a bit two-faced & will do what they can at the time to better themselves! Quite possibly, at the expense of others!
Bob: "Well that's odd, because my sister has just seen her in town with a group of girls, she seemed fine & looked like she was having a good time!"
Rita: "What a Sly Maguy! I can't believe she'd do that!"