If she arrives on, for example, 1 April 2025 then she can stay for 3 months from that date.
I can't remember what subclass visa I had for Australia but mine expired on 3 Oct 2024. Exactly same conditions re 3 months per entry. I landed on 30 Sep and left yesterday, 5 Oct. No problems entering or leaving.
the 180 days in Thailand residency has zero impact on tax treatment in the UK on UK sourced income. As an fyi for you, you can be a UK tax resident from as little as 15 days in the UK during the tax year.
I'm not too bothered about your accountant and whether he does or doesn't have overseas clients, I'm merely commenting on your comment about UK residency and the minimal impact it has on UK sourced income as the UK retains primary taxing rights on UK sourced income irrespective of one's residency status in the UK.