Your brother-in-law might genuinely not be guilty of ABH. Yes, I know that it doesn't seem like it to you but I'm trying to look at it from the way that his solicitor (and the court) will do.
A black eye is consistent with a charge of 'common assault'. See section (vii) here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#06
Someone is only guilty of ABH if they inflict more serious injuries, such as broken bones. See section (ii) here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#09
It's likely that his solicitor will seek to show that your injuries only fit into the type shown in the first link, above. Conversely, the CPS solicitor will seek to show that they fit into the second category (possibly because they include 'extensive or multiple bruising').
I can see no purpose in you consulting a solicitor, since it's the duty of the CPS solicitor to try to obtain a conviction.
Chris