If he uses EAD to work for other company, the sponsor company might pull the plug on his 140 and with that his EAD will also be revoked. He will have no option but to leave the country.
Also if he is working on H1, using EAD to work somewhere else parttime ot full time will invalidate the H1 and he will be at a risk in case if GC is denied for any reason.
Better stay put till atleast 140 is clear.