So either you persuade someone the benefits of what you are proposing (and benefits for the customer not benefits for you or your CV), or do it the way the customer feels comfortable with, or don't do the job.
after all its the customer paying the bills, its the customer that will have to use your app, and possibly maintain it over the years should you move on.
there is no reason why you can't use an open source db like MySQL.... heck I wouldn't be surprised if you could use SQLite within Access. If you are competent in db Design then using something like access shouldn't be a barrier. It could be if the customer wants to deploy the sales leads to say remote devices, or wants to support significant numbers of concurrent users. but again access front end talking to a server back end isnt a problem.. its access front end talking to jet thats a problem.. and Im guessing you'd have similar problems with SQLite
But at the end of the process.. is you preference for Ruby or whatever down to you improving your CV or of genuine benefit to your customer. If your current skill sets aren't compatible with the job in hand then either you have to a pass on the job or find a work around.