I know a girl and boy who pretended to be married to each other just to get the girl in England. Know the girl has been granted indefinite leave to remain in UK. Should I report this to home office

You should do what you feel is right.
Would I? Not unless she was harming those around her.
The planet is full of artificial boundaries drawn by people. Granted countries provide benefits for their citizens that poorer countries don't, and taxpayers pay for them, but are you one of those taxpayers? Do you pay in more taxes than you take out in social programs? Have you never broken a law yourself, or taken advantage of some free program you weren't entitled to? Have you ever thought of what it would be like to wake up some morning and find yourself in Somalia or a tiny village in Mexico or the middle of a war in the Middle East? Bombed out home, no food or water, the police take what they want from anyone they want, or the revolutionaries, or the gangs roaming the streets, or the drug lords?