Most insurers offering short term car insurance
do not accept Australian/USA/Canadian licences I'm afraid. There are only a few companies which does temp cover as well so the chances of you finding that one company which will insure your friend's husband is going to be really slim and perhaps very expensive as well.
You may want to ask your friend whether her current car insurance covers her (or her husband) when driving abroad. If it doesn't, then it might be easier for them to include car insurance cover for driving abroad than you looking for it in the UK.