Hate em. I am a sysadmin not a developer, but I always do some light to heavy dev in support of my environment. In my experience if you are really that good you can let the ideas and knowledge flow. Only half of what you say will sink in and that's only if the listener has the conceptual framework to remember the facts in context.
If they hired the right guy (or gal), he may actually be able to keep up. This will be obvious when you are peppered with smart contextual questions. Rejoice! Everybody is different, but there is a good shot you have an ally for life. Build enough of these and everybody will call you first for the interesting stuff.
It does sound like you have a bigger problem. Sounds like your client does not realize what an open ended thing they are asking you to do. If they do, can't hurt to clarify. If they don't get it, ask them what they are willing to pay for. Look at the longevity of training one guy vs improving docs. Consider the training and intelligence of the individual. You may need to spend ALOT of time with him. Consider the medium. Email is more defensible and cheaper than phone help, which is cheaper than in person visists, which is cheaper than full time in person training. Offer some cheap email responses for a period of time. That helps you to keep track of how well or badly this fellow is doing. Also ask them how you proceed if you have difficulty communicating with him.
Once you and your client knows what they actually want from you it won't be uncomfortable, perhaps it will even be fun. You don't really know something until you teach it.