From the employer perspective
They should provide a P45 in a timely fashion and whilst I don't think there is a specific definition of this, generally accepted timescales are at the last pay date (or just after if there was an issues with the last pay), or on date of departure.

But failure to provide a P45 at all is actually a breach of PAYE Regulations. There is no specific fine for it as far as I'm aware, but it could be picked up on a compliance check and then he could be fined up to £3k under not keeping proper records.

So might be worth mentioning that fine as it might focus his mind to tell you when folk have left!!

This trade is the worst for such, but given the huge turnover of staff seen in the sector I do sometimes (but not very often, anyone might think I'm going soft!) have some sympathy.

From the ex employee perspective
I would always pop the correct date on the P45. This because if they did have another job that was the one on a BR rate, then by not notifying HMRC of the leave date means they could be overpaying tax there and therefore overdue a rebate. So you could be doing them a favour, rather than an injustice.

On the other hand, it's almost April, any tax refund due should be picked up via the end of year processes and your p45 if it was dated today. But I would still date it when it actually happened, to represent 'fact'

A compliance check could competed eg if the ex employee had phoned to query their tax or if they notify HMRC of not receiving a P45 or just via the random checks they often make on such a sector.