It's going to depend on which countries they're visiting. Commonly in Europe, you pay for your local phone calls, so there are many ISP's that are free and get commision on the call rate.
In the UK, dial up ISP's don't charge, some don't even require usernames. For example, when I'm back in the UK I use
http://www.freewire.co.uk/
To manually setup a connection simply dial the local rate access number 0845 028 0088
with any username and password. You must ensure you don't have any proxy server or specific DNS servers listed. Set everything to server assigned.
Basically I set my dial up to the number and use 'guest' as both username and password.
there are other ISP's like this around europe. Do a google on free isp europe and you should come up with something.
Another alternative is AOL which has links everywhere.
J.