Assuming its an Intel Mac they should be able to run under Parallels (which I do) or possibly install Wine and run it there.
If the application is all GUI and developed using Nomads it may be possible to run it under iNomads however there are some limitations such as no local drive access and local code execution that are not available while running in a browser.
Now depending on the application and the portions they want to run you may find you can implement the required functionality using Webster+. It supports running reports, charts, and queries using existing program/query definitions. Also the file maintenance generator can generate screens for the Web both maintenance screens and queries.