Also, they can choose from tons of Hostels spread across the city like Zostel, GoStops, Moustache Hostel and more.
There are plenty of places to explore in Delhi. If they wanna experience the heritage of Delhi, go for tombs, monuments like Qutub Minar, India Gate and the likes. Other than that, they can go to Champa Gali, Janpath and CP for shopping.
They should first explore Historical sites then Malls and Markets. For affordable stay, they can go to Paharganj near CP.