I wouldn't do this.
If he wants to either scatter or bury the ashes, you are looking at about four pounds worth for a woman. It's quite a lot. If you bury them, and you will need a large hole as anyone who uses bone meal will attest, they will not necessarily biodegrade. In fact, you might just end up with large clumps of ashy salty rock just under your top soil; this can kill plant life as ashes can have very high levels of sodium or phosphate in them.
The more environmentally friendly way in terms of soil and plant life to dispose of ashes is to scatter them over a very large and hardy area. With a garden, however, well, consider scattering four pounds of fine grit and concrete over your garden that will not sink into the soil. You may also find bone fragments, and maybe other small pieces of metal that the crem processes missed (fillings etc). I certainly would be very unhappy with pets or children playing in the vicinity for at least a year, and I would hesitate to garden in the area.
Depending on the weather around the time if you scatter, you are also risking ashes going where they are not intended to. You don't want your plants, your garden furniture, your windows, your paths to be covered in a layer of ash.
I personally think the request is a incredible imposition upon you, and the gentleman needs to think carefully about what he is asking and you need to consider about the implications for the future. He could, for example, get very upset if something changes in the garden. He may feel he has a right to visit. I am also concerned about the notion of "a ceremony" in the garden; this is not someone who just wants to scatter a tablespoon under a rosebush for old times sake. It sounds like they want to use your garden in the way you would a formal remembrance garden.
It is worth pointing out that in my borough, all ashes now have to be interred in a suitable and licensed area precisely because of the environmental impact of scattered and buried ashes.
And, yes, due to some weird quirk of fate, I have quite a lot to do with human remains and policy. 