Hello, HS12, and welcome to the Grammar Exchange!
I have a friend than whom my father speaks English better
is totally ungrammatical.
"than" needs to appear after the adjective in comparative degree.
If you want to use a relative, you can choose one of these:
- I have a friend whose English is not as good as my father's.
- I have a friend who doesn't speak English as well as my father (does).