None of this is going to sound great to you, but it is the harsh reality.
-- She chose one of the opaque third-party vendors and therefore it is her travel agent. She needs to deal with the TA. DY won't touch the ticket.
-- There are few options as a practical matter. DY does not interline with other carriers, so she is stuck with DY.
-- A bit of research is in order. Why was the flight cancelled? Is there a flight the next day or perhaps a day earlier. Might she prefer one of those if available?

She has three choices:

1. Full refund to her original form of payment.
2. Rebooking under similar circumstances. Some argue that this means that DY must purchase a ticket for her on another carrier, but there is no holding to that effect and as DY has no interline agreements, it isn't going to run out and purchase a cash ticket without a court order.
3. Rebooking at later dates of her choosing.

There is an argument that DY has a "duty of care" to pay for her hotel for the 2 days, but it won't and unless she is prepared to sue, she won't see the money. There are claims agencies which will handle this for 25-33% of the loot, but that is a tough one for a couple of nights in a hotel.

The best bet here is her travel insurance. Either to pay for her return as scheduled on another carrier or to pick up the hotel tab.

It is water under the bridge for OP, but this is a public board. Things that look cheap often turn out to cost more in the end. If you are going to book an LCC using a sketchy website, then hedge with really good travel insurance which you've carefully researched.