My husband, who is Indian, travels there often (4-5 times a year) for work. He's been to Delhi and Agra, but he only had ideas for Delhi. Here are his suggestions:
"I only know Delhi... they should try Chandni Chowk - it's the ancient city's centre and market place. Extremely busy and crowded. India's largest and oldest mosque is there for visiting. LOTS of different foods and clothings and souvenirs, etc. Another good shopping place is Connaught Place. This is newer and not as overwhelming. It's still not like a mall or anything, but not as overwhelming. And they have better restaurants there.
That's about the only sightseeing I've done in Delhi. I've always been too busy to do anything else and would just eat at wherever I was at a meal time...
What's their budget? Are they looking at high end hotels? If so - Leela Palace in Gurgaon (kinda a suburb of Delhi - close to the airport) is an excellent hotel to stay at ($300+ a night). The Hilton at Janak Puri (another kinda suburb) is another good one. The Lalit is another good hotel (the latter two are slightly cheaper and you can get a room for $200 or less a night). They're all 5 star hotels.
Ooh while in Delhi, they should check out some tourist sites in Uttar Pradesh. I don't remember exactly where in UP, but it's an hour away."
Hope they have fun.