Far-fetched means unlikely. If you say that someone's idea or proof is far-fetched, you are saying that is unlikely to be correct.

Far afield means distant or wide-ranging. This doesn't quite fit either, at least to my American ear. I expect far afield to describe distance traveled ("I've gone far afield of my home") or a difference between two topics ("I started in biology but now my studies have gone far afield of that, into literature and mathematics").

Perhaps the best adjective I know for this is roundabout, which is an antonym of direct:

circuitous, indirect < had to take a roundabout route >

I might say:

This is quite a roundabout solution; maybe you can think of something a bit shorter? (or: ...something more direct?)

Far-fetched means unlikely. If you say that someone's idea or proof is far-fetched, you are saying that it is unlikely to be correct.

Far afield means distant or wide-ranging. This doesn't quite fit either, at least to my American ear. I expect far afield to describe distance traveled ("I've gone far afield of my home") or a difference between two topics ("I started in biology but now my studies have gone far afield of that, into literature and mathematics").

Perhaps the best adjective I know for this is roundabout, which is an antonym of direct:

circuitous, indirect < had to take a roundabout route >

I might say:

This is quite a roundabout solution; maybe you can think of something a bit shorter? (or: ...something more direct?)