Briton Richard Collins, 35, was killed by Uthain Duangnoi who admitted the crime - yet remained free on bail until this week.
His initial sentence of 15 years was halved on appeal, but Yvonne Hart, the mother of Richard Collins, was shocked at the shortness of the sentence.
This week, after successive appeals by both sides, the Thai Supreme Court ruled in her favor, the Hereford Times reported.
''I am absolutely delighted that our appeal has been upheld because it's been a long, hard struggle,'' she told the newspaper. ''I am just so thrilled.''
Ms Hart said she is still disappointed with how Britain's Foreign Office has treated her family during the eight years.
She will be joining 100 other people whose relatives have been murdered abroad at a demonstration at the Foreign and Commonwealth Office on October 9.
Richard, who loved rock climbing and poetry, said the newspaper, was killed when he returned to Krabi with his Thai girlfriend.
The couple checked in at the Greenery Hotel after being picked up by their friend Sunitsa Sririrot.
But when Sunitsa asked Richard to drive, her husband Uthain Duangnoi flew into a jealous rage when he saw the British man behind the wheel and he stabbed Richard to death, the newspaper said.