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Normally, any plastic or lead anchor will work well in masonry. However, a hammock's load is partially vertical and partially horizontal, so there is a greater risk of the anchor pulling out.
With a sturdy stone-and-cement wall, it is usually easier to install the bolt between the stones in the mortar, as many stones defy easy drilling! So given these two problems, the best solution would be to use 1/4" or 3/8" eyebolts. They would have to be set into the mortar at least a two inches. The hole would have to be large enough to allow a nut installed on the end of the bolt to fit easily into the hole. This nut will keep the bolt from pulling out.
A fast setting anchoring mortar would be used to set the eyebolt in the hole. This should hold the hammock in place safely provided that the wall is sound and the rest of the mortar is solid and not crumbling.
If confidence in the integrity of the wall is low, though, it might be better to suggest the customer purchase a hammock frame!