@TatianaBis, It's definitely not dialect.
I am not confusing anything. DD renders to TH like in The and That, not like TH in Thing
I won't say the name as it could possibly out the person and out me.
It is something like Beffan for Bethan or Geffin for Gethin. and is quite possibly a speech impediment. Person has been teased about it since childhood but is so likeable that no one cares. They pronounce it the way they do, speech impediment or not. It has nothing to do with accent.
Not sure if my two friends have speech impediments. One might. The other speaks a bit lazily.
The person who can't say J has an impediment.
Many young children struggle with TH. They usually grow out of it, are corrected or have speech therapy.
As I said, just because you haven't encountered it doesn't mean it doesn't happen.