There is a direct bus from Liberia to Puntarenas along the Panamerican highway (not a bad road). It drops you at the bus station and you get a $4 taxi (and there are always taxis there) to the ferry to Paquera (1 hour on the ferry - beautiful, relaxed ride). In Paquera, the Montezuma bus always times itself with the ferry, so you have the option of the bus or grabbing a taxi (expensive, but they are always there).
The other option is to get a shuttle, which will also take you all the way, or you can go just to Puntarenas, and pick up a bus/taxi on the other side, but you still get the ferry ride
The bad roads mentioned are only if you try to drive down the penninsula instead of the Panamerica Highway. All buses or shuttles will take the highway and turn off for Puntarenas - all good roads
I assume, they have booked the Liberia flights already (who can argue with free!).
It is really not a diffcult journey, but the bus will take all day (it stops everywhere, even though it is called 'direct'), the shuttle much less time but costs more.
Another option is to fly Nature Air from Liberia to Tambor and taxi from there (taxi is about $40-$45)
The current ferry schedule from Puntarenas - Paquera is
The ferry schedule does change - The number to call to check is 26612084.
If they go with Nature Air - call their toll free US number. They are a great company, but their website has issues : )
The journey from Liberia is really not difficult, nor is the one from San Jose, but it does help for them to know some details beforehand. A lot depends on how much time they want to spend travelling and their budget
I hope this helps