Some of these services don't allow commercial use of the font, so read the small print.

There is a nice iPad app, that may be can help him, if he has an iPad. Is called iFont Maker.
http://2ttf.com/

I really love it and it works very well.

and you can convert easily into a ttf file.

I think for what your client is after you need to have it professorially made.

Perhaps your client could make the font on the iPad ap, and then you or someone else could fix up the metrics, tweak the glyphs a bit, if required and add a few alternates and charge for that part of it.

@Simon: YourFonts.com says on their site that once the font is digitised you'll be able to sell it, so commercial use is not an issue.

@Guiseppe: I know iFontMaker, but nor he nor I have an iPad (yet!)

@Russell: I think your right (except for the iPad part which just isn't possible at the moment). I think I need to instruct my client to write some texts (Quick Brown Fox et cetera) as well as seperate characters, then scan it and rework it in FL.

A lot depends on how cursive the writing is.
These days, a good cursive handwriting font is a huge undertaking, due to the amount of OpenType contextualization that is now employed. However, if the writing is more like printing, with discrete letters, then it's a lot easier to make.
Perhaps you could suggest your client writes in more of that style?

and what about a wacom tablet?

The problem with writing on a tablet is getting used to looking at the screen rather than ink line coming from the pen nib. It takes a while to get used to. I've used one for years and I still can't do anything that resembles my actual handwriting with it.

I've asked my client to provide me with a sample of his handwriting so I can see if it's all discrete letters or cursive or a combination of the two.
To be continued.