Question : Notify me greyed out in find my friends 2.0 iPhone 4

I have a 4s and when trying to get fmf arrival/departure notifications from an iPhone 3GS, the notify me button is greyed out. It does work the other way around though (ie the 3GS can get notified when my 4s arrives/ departs). Both phones have fmf 2.0 an ios6. I'm thinking this is a limitation in the 3GS GPS, but can't find any documentation. Can anyone get fmf notifications when tracking a 3GS?

Thanks.