Photo from Mt Adams by John V. Christiansen around 8:32 am local time on May 18, 1980 & published in a 1981 National Geographic magazine. https://old.reddit.com/r/AlternateAngles/comments/bzvxin/photo_of_a_hiker_watching_the_eruption_of_mt_st/
Found on Reddit. https://old.reddit.com/r/interestingasfuck/comments/coh7nt/1980_eruption_of_mount_st_helens/
I'd like to credit the reddit post I found this on but I lost the link and had just this open in a new tab.