Tom,
Marshalls brought out a product very much like this a couple of years ago, called M-Joint. It was ok,ish but not that great to much depended on width and depth of the joins. Personally I wouldn't let a customer tell me what to use, and also the fact that you say some pointing is missing, will the colour of this stuff match up? or is it going to look like a bodge job? I would clean it, and recommend they get a paving guy in to do the re-point.