See chapter 5 of RDR1 here for information on domicile.
Hi again.You should refer to HS283 as part of this answer.The fact that the couple intend to move back to the UK means that they are really still UK resident for tax purposes. In order to escape Capital Gains Tax in the UK, they would need to stay abroad for at least 5 years and sell a property which has not been their main home for the entire period of ownership plus a maximum of the last 18 months of ownership whilst abroad.Having said that, from 6 April 2015, non-residents will be taxed on gains made on the disposal of UK residential property but only on the difference between the sale price and the value of the property on 5 April 2015, again unless the property has been the main home for the entire period of ownership plus a maximum of the last 18 months of ownership when they were not living there.If the property has been your friends' main home throughout their ownership of it, then any gain will be exempt from CGT under the main residence rules provided it is sold within 18 months (36 months for disposals by 5 April 2014) of their having moved out. If the sale takes longer, then part of the gain may not be covered by main residence relief but letting relief would cover or reduce the remaining taxable gain. See the examples on page 4 of HS283. Information on letting relief can be found on pages 6 and 7.You should read the notes on divorce and tax here. If the couple separated and one of them moved out of the property and it wasn't sold for a period beyond 18 months after that, then part of the gain of the spouse who moved out may be taxable.I hope this helps but let me know if you have any further questions.