Unlike the Hawiian volcanoes Mt St Helens spews mostly ash. It is layers and layers of ash. The 1980 eruption was not even the biggest. As you can see below it is a frequent eruptor and is actually younger than the pyramids. Many of he eruptions likely suffocated BF and other animals living in the area. My trips to the mountain often include walking the streams and creeks seeing what is eroding out of the ash. For a volcano that spews basalt, anything that survives the 2000 degree temperatures would be encased in solid basalt. But the ash is soft and carved away every spring. No digging required but an incredible amount of luck to happen to be there when something erodes out. Areas now with BF populations are likely to have been that in the past and got some suffocated by the ash from some eruption. If BF buries their dead in some safe and stable place, it takes a natural calamity to kill and cover them to allow them to be found. Volcanoes, land slides, avalanches, are all such events. I think this protocol is more likely to succeed in a skeletal discovery than most anything other than shooting one.
https://pubs.usgs.gov/gip/103/