First and last month advance rent instead of a Deposit?

I was talking to a friend of mine at the weekend who has one property she lets out and I asked which deposit scheme she uses.

She said she doesn’t take deposits, claiming it’s not necessary if you apply the “first and last month rent up front” method.

Although I’ve worked in the lettings industry I’ve never heard of this before – is she walking a dangerous line if she ever wanted to issue a S21 if she’s never provided any Prescribed Information, which she won’t have if she doesn’t consider it to be a deposit, or risking a punitive fine if she has a savvy tenant?

If it is acceptable, when could a S8 be used if the last month is paid in advance? If the AST goes periodic how is the last month determined?

Alternatively, is this an old practice which is now no longer acceptable and she urgently needs to be updated?

I look forward to reading your thoughts

Regards

Tilly Mint