It's a fairly new game, with 300 5-star reviews. Seems to be an addictive game looking at the comment from OzBargain.
Pricetracker here
Requires iOS 7.x+
Seems that there is only one In-App purchase available (10 keys $1.29).
…which isn't required to play the game. To buy it, he'd need to know your Apple password, or have exactly the same fingerprint as you (if you have TouchID activated), so I'd say you're pretty safe :)
Via TA on OzBargain
