very simple question, unfortunately can't nail down the answer simply because there's no written agreement in place.

in short this type of arrangement, anything can happen.

As for your deposit, if your friend can't find future tenant towards the end of the 6 months tenancy. Be prepared to lose that deposit (which normally is x3 rental). Your landlord also quite smart.

In my view…

Your friend’s landlord has converted your friend’s deposit into an obligation. This obligation entails that you find another tenant, who in turn will pay to the Landlord a deposit and the Landlord will reimburse you back. If you fail in your obligation to secure a future tenant, your deposit will be forfeited.

Traditionally, deposit for tenancy is intended to secure the performance of the tenant in paying his/her rental. It is not intended as a further obligation (i.e. secure future tenant or repair damage to fixture and fitting etc…).

Conclusion:-

1. don't take up the tenancy, its just too iffy.
2. if desperate, take up and your friend can try and ask landlord to put in writing (which i doubt he will do so).