Waiting on what bank they use - hopefully it's ABA or Acleda and we'll go from there. It unfortunately needs to be very simple and straightforward. Sorry another question - are the service staff at these banks bilingual khmer/english by any chance? My wife can be on the phone with her dad but she won't understand technical banking terms in khmer and she can't read it. Of course dad doesn't speak english so attendant needs to speak khmer.
The form that has to be filled in the bank is quite complex and needs to be done in English. Your father-in-law will need the recipients bank details including SWIFT code, ABA No. etc. If you need it simpler you should perhaps reconsider Western Union. Bank staff mostly speak some English as well as Khmer - most are helpful, some are not.