Given the lack of details, it's hard to give a more specific answer because it can be ranging from 'tried to played politics, sucked badly, fired back' to 'was pushed out by someone manipulative'. Also, there's usually more behind the 'just internal politics'.
Is it a bad reputation for him to be terminated because of internal
I'd probably not focus on the politics bit on the interview unless pressed really hard. Some might see it as a red flag because not everyone likes people involved in the politics or various other reasons.
How to explain to his future interviewer?
Something neutral as suggested by HLGEM such as "I got caught between forces beyond my control" will be enough, usually.
Can he explain the details of what happened to him?
There's no need to get into details because you might get too emotional, interviewer can misinterpret that, etc.
So don't lie, obviously, but there's very little point getting into too many details as it will not benefit you.