Did he not say he's sailing on Lake Geneva? A USCG approved lifejacket might be hard to come by there.
Actually, what you want is really a buoyancy aid - not a lifejacket. A lifejacket will keep an unconscious person's head out of the water if fitted properly and inflated. A buoyancy aid will keep you afloat, but possibly face down. The thing is, its almost impossible to swim in a lifejacket and can really hamper getting up on to the daggerboard to right a capsize. A proper water sports buoyancy aid will allow you much more movement both in, and out of the water.
Lasersailor is right about the tether - bad idea in a dinghy. Ideally you should be sailing with safety boat cover - particularly as you're learning. Join a sailing club or go to a sailing school approved by your national sailing association. Also make sure you and your crew practice and can both carry out a figure-of-eight man overboard recovery maneuver. This is a primary requirement for the UK Royal Yachting Association level 2 certificate - the lowest level at which someone is certified to be able to sail a dinghy without supervision.
http://www.apsltd.com/Tree/d297000/e294573.aspLifejackets aren't like the generic orange things you keep stowed aboard a boat anymore.

The difference is that USCG Lifejackets are rated to provide at least a certain amount of buoyancy. Buoyancy aids either can't pass the USCG tests, or have yet to be put up to the tests.
Anyway, should you choose to pick Buoyancy Aids to wear, you would have to store on your boat 1 USCG approved lifejacket per person using the boat. Since wearing a USCG lifejacket is no longer a problem, and you need it aboard anyway, don't bother with Buoyancy Aids.
Not to mention that any outfit you race with will require a USCG approved jacket.
If you are worried about it, buy a brightly colored lifejacket and a rescue whistle. Attach the one to the other and you suddenly become leagues easier to find should the absolute worst possible set of circumstances to ever happen does actually happen.