you don't want the thread features to be too small in aluminum (especially the soft grades used in masts), but in a larger bolt like 8mm the fine threads would be coarse enough. My reasoning is that finer features are more easily eaten by corrosion or damaged in other ways, but in a thin wall you need at least a few turns. To get a good thread in a thin wall tapping by hand - especially with fine thread in soft material - is difficult, the tap must be started square and kept that way until the thread is at least partly formed. It is all too easy to start just a bit crooked, a tapered hand tap will then straighten itself as it goes in, at the cost of the thread profile. He may be able to clamp the fitting in place as a guide.

There are blind threaded fasteners that might work as well (finding them in a remote location might be difficult). Gluing the fitting on as well might help, you need to sand it within a half hour or so of gluing to get the oxide off so it will bond to the aluminum.