Just explain it in plain English:
"Using this font on the web requires an additional license, which is [$xxx per year / $xxx per 10,000 page views / not available for this font].
"Consequently I recommend that you [use some other font instead / pay $xxx for an additional license for 12 months]."
It's really not that hard to understand that using a font on the web requires an additional license. You don't need to explain the ins and outs of font licensing, etc.
One thing you should probably note, too, is that even for non-web work, you probably still need another license. The client's font license won't cover use of that font by you (even if they have a multiple-desktop license, it is unlikely to cover usage on desktops not owned by them).
It would be appropriate to figure out the fonts you'll be using and account for their licensing costs, both desktop and web licenses, when you draw up your initial quote/estimate for the client.