Sharing between Iphones

My husband had a IPhone 4S which he gave to me when he got his new IPhone 5. Before he gave it to me he loaded Motionx GPS on the 4S. Now that I have the 4S, I want to add the Voice Guidance, but the app will not let me purchase it. The message I get is, the app was purchased by another Apple ID and to get the Voice, I must purchase the app again. Is there anyway around this? It doesn't give an option of logging in with his ID. I've also tryed syncing our accounts with Home Sharing and that doesn't work either.

Help!