Speaking of myself who would do this very thing until about nine years old,I thought my reasons would help people understand what was going through my head at that time.
I was the youngest of four ,born in 1964, we were very hard up and us kids knew that ,and knew not to ask for things,even clothing ,which were growing out of,and as a girl at that time was aware of pretty things and being attracted to them ,but also knowing I would never have it or anything like it.our mum was never at home as she worked long hours at a supermarket and our dad was mentally ill and had low paid work ,which he would lose by acting odd.
I know I felt left out ,of mum being there,and trying to compete for attention with my older siblings.
When visiting friends if they had a pretty item ,or something I admired ,I would take it , then hide it,and then feel guilty,but I couldn't stop myself.
I would secretly look at this for hours and put it back into its hiding place,of course I was eventually found out.
Please don't stop this child visiting, children do not steal or lie if they are happy,this is a sad child!
I became a secondary school teacher and also found this to be true,what I found works is...
When the child is over,have a loud conversation between yourselves along the line of " it's very funny but some things have gone missing,we will have to be extra good at keeping a eye out to see what happening,address this to all ,this will indicate that you are aware of bits going missing.
I am sure that this will work ,it would a shame to part a good relationship,try to not cross with her,
Having experienced this myself and taught children from 5 to 15 ,it's mostly lack of something that makes a child do this.
I know kids these days have more,but they can still lack,time, patience,love,or Indeed pretty things.