Thank for the historical lore, but setting is a DM matter. So he may do some tweak.In historic lore? At least for the FR, Yes they do become powerless.
I'm not up on other settings nearly as much for this, but given that FR is now the default setting I think the older lore applies to the setting in general.
When Bane died in the Avatar Crisis, his priests lost their spells until they switched to worshiping another god. Most jumped to Cyric or Iyachtu Xvim. Same with Leira when she died, though being priests of illusion and deception, they pretended. Some started to get their spells from Cyric as well.
So I would see no problem for a good fiendish warlock, explaining he made contact with fiendish energy by some ritual, bloodline, possession,...
I prefer letting freedom to players than babysit them,