How do you set that up on an Android or iOS device?

Use a TOTP for a second factor of authentication and give it to your friend abroad.

So you have to contact your friend every time you unlock your phone screen?

That's what your lawyer would say is the case is the impression I got. Or, more seriously, perhaps that could be a toggle. Similar to how if you hit the panic button on iphones it no longer takes biometrics until you give it the code? That might solve for ease of use.

Lawyers aren't allowed to lie in court.

Hitting a panic button would mostly likely qualify as evidence tampering, disruption of investigation, or evasion of justice.

These were non-uniformed undercover police that intercepted the guy. In this scenario it's very plausible to think you're about to be robbed, and hitting a 'lock device until I return home' panic button could never be considered any of those things given the circumstances (under UK law, anyway.)