CGT in the UK doesn't work in the same way as it does in Australia where the assets of an immigrant are valued at the time that individual arrives in Australia so that only the gain which accrues since their arrival is taxed. The whole period of ownership is taken into account in the UK so if your friend was UK tax resident in the tax year he or she sold what was their main home in Australia, there may be CGT to pay in the UK.
Take a look at HS283 here, in particular Example 9. As a proportion of the whole gain, that part covered by the owners's occupation of the property will be exempt from CGT as will that part of the gain covered by the last three years of ownership. Letting relief will also be due for any period not covered by the owner's occupation of the property and the last three years of ownership. That will be the lesser of:
2 the sum of the gain covered by the owner's occupation of the property and the gain for the last three years of ownership and
3 the gain for that part of the letting period not covered by the periods in 2 above, (probably 1 year of the last 4 years of ownership)
If there is a UK liability to CGT, had any CGT paid in Australia on the same gain, it would have been deductible from the UK liability.
I hope this helps but let me know if you have any further questions.