How can i deal with a good friend that steals...?

well i jus clicked with this girl when i started college and we are like best pals. but when i went to a shop with her..a low budget shop..she reached for a packet of sweets and decided to open it until i automatically moved it away from her and said in a joking nervous voice..what the hell are u doin?? from then i felt lost.. cuz i have neva been in that situation. im 18 very responsible and sensible. its hard but ever since that day i am confused ..its been a while since that day but my mum says dont go shopping with her...because you do not want to get into trouble because of her..i am scared of inviting her to my house because i think she is going to steal something. my sister was confused because she thought my friend was wicked and a lovely bubbly person. i mean its only sweets which makes it even worst because she is quite weak to go to the lowest point to steal sweets. what should i do?