How to wipe an iphone 4 of its data without someone knowing?

I'm sort of being blackmailed by a coworker who has some pretty serious incriminating evidence on some things I've done in the past. Nothing illegal but could definitely hurt my employment and my personal life as well. They have an iPhone 4, and they are upgrading in two weeks. They also don't know how to back their iPhone up, so deleting the data on it would set me free. I was thinking a strong magnet, but it would have to be small enough in size to smuggle in with a back pack. Anybody have any ideas? Btw, their phone is locked and I don't know the password, otherwise I'd be able to do it.

1. Thats destruction of personal property
2. All phones use SSD (which is magnetic proof) aka you would need a super-magnet capable of RIPPING ELECTRONS from the atoms in order to damage the data
3. The ONLY thing you can do (legally) is contact the police and tell them about the black mail… Try and get some proof BEFORE doing it… Like a text or a letter.