Eddie,
I don't have a solid answer, but I think it's common sense that if the marriage is terminated then technically so would the extension based on marriage. But how would Immigration know unless the soon to be ex is vindictive and drops a dime.
There was a related topic on Thaivisa and the consensus was that immigration would unlikely find out.
However, I believe that Bob's earlier reply would be the best advise:
BobHelm wrote:
I have no personal knowledge of the marriage visa but have heard rumours that, once you are divorced & the authorities find out about it then the visa will be cancelled & you will only be given a 7 day visa & must then leave Thailand.
My advice would be that, as he is in the UK anyway, he invests 100 pounds in a non imm O multiple entry visa - reason for entry 'to visit family & friends'.
That way, once he is divorced there is no married person visa to cancel.
I do not think it would be sensible to come back on the marriage visa unless his only purpose was to get divorced & then return to UK.