The main question will be whether he is classified as a trader for tax purposes. If you are classified as a trader then your profits/losses are counted as income and not capital events.
If you aren't counted as a trader then you can account for it however you want within the same account. Say you bought 500k shares originally. Then later on you bought 200k shares. You sell 200k shares you allocate that sell to the recent buy and your original parcel stays in tact. Then after 12 months having passed you sell the original parcel it will be eligible for the 50% discount. You just have to be able to demonstrate your methodology and transaction history.
People use LIFO, FIFO, allocate transactions to individual parcels and so on. There is no right or wrong way. It just depends what suits.
