Agreed. It is probably in the fluid line, just a matter of checking it all out. If the master checks out ok and you bleed the line and it works at first then does the same thing, you will need to replace the slave cylinder. Remember to disconnect the feed line and 2 mounting bolts on the slave, reinstall the new one and proceed to fill up the master cylinder again, and then start your bleeding process all over again. That's the part chicks know about.
Are you telling him, even if he's not willing, he should be able to get by with doing this once a month?