It probably would be nothing more than a benefit to the tree, as grim as it sounds. "Potash" is one of the main components in fertilizer.
I would incorporate the "remains" in the surrounding soil before adding the soil back into the hole in which the tree is planted. A hole 2-3 times the size of the root ball can be quite a bit of soil. When I say "2-3" times the size I mean in diameter, not depth. The depth of the hole should be no deeper than the bottom of the root ball to the beginning of the root flare.
I'm not much a fan of cremation for personal reasons but if I were to be cremated I think that would be a good use of my remains.
Excellent advice, but I think that is a bit of overkill. Having participated in a few private "ash-dispersal" events, I think doing all that work is a lot more work than necessary, and not so good for the solemnity of the event. Human remains don't make that big of a box of ashes, and they aren't ever going to be a health problem for a tree. The symbolism here is more important than the effective disposal of the remains.
If the fruit tree is going to be a dedicated planting for the memorial of that person, just do a regular planting with standard good practices, except leave off the mulch cover. Have it planted, staked, and all the hard work done prior to any ceremony, and it could even be done by the nursery or landscaper that sells the tree.
When it comes time for the memorial service, spread the ashes over the planting ring, then gently rake them in a bit. Cover the ashes with mulch, symbolically burying the dearly departed. A nice touch might be having some water handy to water the tree with, signifying the transfer of their mortal remains to the living tree.
Be sure that the tree isn't going to be planted in a marginally appropriate location. Not much will spoil the lasting effects of Poppa's memorial site like returning to a dead tree. I'd recommend planting at least a 1" to 1 1/2" caliper tree, and then ensuring it gets watered for the first year often enough to prevent dessication during the summer. Planting a tiny tree like a cutting will seem a bit cheap and is prone to accidental destruction by mowers or rabbits/deer chewing it to death. Large trees are more prone to failure from inadequate after-care.