Lost Iphone 4S

evening all

A friend of mine had her iphone 4S stolen about a month ago, she's currently using my old phone, but from time to time when i send her a text the send button is blue instead of green and it sends an imessage? It never comes up 'delivered' though. Question is , on the occasions that the send button is blue, is the lost device switched on at those times? And does the imessage go to the said device rather than my old phone?

Thanks alot
hle