As a person with some conscience, I can't accept that this guy has to silently leave her and find another woman.
Reasons:
(1) They both got married (not clear if consummated) and are purportedly in love.
(2) She is turning out to be a cheater
(3) The guy that she's getting married to is not only being cheated but is getting a bad wife
It's clear that you have been cheated (may not be legally but you know it as a fact). You should not let it happen to any other man especially that guy that she's getting married to. At the same time, you should not get into any legal trouble.
Some of the remedies depending upon your stamina to fight as a man
(1) You enjoyed her for two years, just leave her and her fiance to their fate and move on
(2) Inform all the families involved about your marriage, present the photos if any (to her fiance too) and try to resolve it.
(3) If you have any faint of evidence of marriage, take it, file a case. There'll be counter cases but they won't last long bcos the girl wants to move ahead in life