If she's intending to travel overland out of Cambodia it's much easier via Vietnam than via Thailand.
There's two overland routes to Bangkok from Phnom Penh, neither are easy, one goes west of Tonle Sap, the large lake by Seam Reap (Angkor Wat) and the other east of the Mekong and the lake, via Seam Reap.
The best bet is straight up the mekong on the river boat to Seam Reap.
But from there it's taxi or back of a pickup to the Thai border. Quite a distance. For a few dollars extra she could try getting to sit in the cab.
There is a railway but no trains!
You don't need a visa for Thailand, just a few dollars at the border.
Whereas there are multiple options going from Phnom Penh to Saigon. It's a well recognised tourist trail and it's shorter.
If you go via minbus the driver collects all the passports and takes them for verification while the passengers wait in the cafe!
If she's intending to fly to Bangkok, tell her to try haggling for the price of the ticket. (at Phnom Penh, that is, not at Heathrow!)