This is how I finally solved it.
OPTION 1:
Get a copy of the following files from a computer with a working setup of the SQL Anywhere ODBC driver:
dbodbc8.dll
dbcon8.dll
dblgen8.dll
Create an install.cmd file with this:
COPY %~dp0dbcon8.dll %SystemRoot%\system32\dbcon8.dll
COPY %~dp0dblgen8.dll %SystemRoot%\system32\dblgen8.dll
COPY %~dp0dbodbc8.dll %SystemRoot%\system32\dbodbc8.dll
regedit %~dp0SAS8.REG
pause
Create an SAS8.REG file with this:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\ODBC\ODBCINST.INI\Adaptive Server Anywhere 8.0]
"Driver"=hex(2):25,00,57,00,49,00,4e,00,44,00,49,00,52,00,25,00,5c,00,73,00,79,\
00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,64,00,62,00,6f,00,64,00,62,00,\
63,00,38,00,2e,00,64,00,6c,00,6c,00,00,00
"Setup"=hex(2):25,00,57,00,49,00,4e,00,44,00,49,00,52,00,25,00,5c,00,73,00,79,\
00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,64,00,62,00,6f,00,64,00,62,00,\
63,00,38,00,2e,00,64,00,6c,00,6c,00,00,00
[HKEY_LOCAL_MACHINE\SOFTWARE\ODBC\ODBCINST.INI\ODBC Drivers]
"Adaptive Server Anywhere 8.0"="Installed"
Run install.cmd
This will work when Windows is installed in C:\WINDOWS, otherwise edit the registry entries.
OPTION 2:
If someone left a copy of the Powerbuilder CD in the client's computer, look for the folder asa801runtime and install that to get the ODBC driver working
EDIT:
Example connection string with default username/password:
Driver={Adaptive Server Anywhere 8.0};UID=dba;PWD=sql;DatabaseName=base;EngineName=gestion;CommLinks=TCPIP(HOST=GRA06:2638)