I'm an actual extreme metalhead who has listened to death metal and black metal, and these 2 genres' subgenres, for almost 17 years now since I was 15.
The reason he found the music tame is because heaviness is the objective of death metal and its subgenres, not black metal and its subgenres.
The objective of black metal is to create an "atmosphere", a mood, feeling or emotion, or an "emotional landscape/world".
Deicide is death metal and Venom is heavy metal that influenced the creation of the black metal genre.
Maybe your friend should have listened to other extreme metal genres like death metal and its subgenres brutal death metal and slamming brutal death metal, deathgrind, goregrind and more.
But if he doesn't want to be into metal anymore that's fine.
There are three non-LHP, Christian black metal bands I know of,
ANTESTOR,
BATUSHKA, and
HORDE.
These three bands in particular are pro-Christian and safe for Christians to listen to.
There is the pro-Christian death/doom metal band called Paramaecium.
There is lots of pro-Christian metal of most metal subgenres out there, non-metalheads just are unaware of where to look for it.
For everything metal, please talk to me and/or use metal-archives.com , the metal encyclopedia for all metal bands of all genres and subgenres.
These are the results for Christian only lyrically themed metal bands, minus all the anti-christian lyrically themed bands. If you want to get into Christian metal, please check out any of the bands below.
The non-extreme genres of metal are heavy metal, speed metal, and power metal, and traditional doom metal (slow heavy metal, and not other doom metal subgenres because other doom metal subgenres become extreme (like funeral doom metal) and mixes well with the extreme metal genres, like death/doom and black/doom).
There is a pro-Christian death/doom metal band called Paramaecium.
Both heavy metal and traditional doom metal were started by the first heavy metal band Black Sabbath as you already know, and the entire doom metal genre was inspired by Black Sabbath. Black Sabbath are classified as heavy/doom metal.
These non-extreme genres have singing only, and don't have death metal growls or black metal shrieks and screams.
The genre power metal is the pleasant sounding "happy metal" genre with happy melodies, inspired by the nice melodies of Iron Maiden, and is the "safest" metal genre to listen to for non-metalheads and Christians.
Personally, I would recommend you the pro-Christian neo-classical power metal band from South Korea called JesusBand, but I can't find their material.
Check out these Christian power metal and doom metal playlists on YouTube.
Christian Power Metal
Power Metal Christian
www.youtube.com
Christian Doom Metal
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Christian Doom Metal albums
CHRISTIAN DOOM FOR EVERYONE
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Hope this helps. If you want more Christian metal, let me know. Enjoy and have fun!