The Find My Friends app can send notifications to an e-mail address when the iPhone arrives or leaves a location. Polling the IMAP mailbox for these messages, it should be possible to identify whether a user (or more specifically an iPhone) is present or not.
If FHEM knows whether someone (or more usefully everyone) is home or not, rules could be created on this basis. Example. If( "no one home") { set TYPE=FHT desired-temp 17.0 }.