Ah, the plot thickens.....
Well, if your friend cant prove it was okay before he sold it, then he may have issues. If however te guy he sold it to used it with a 40amp esc, then I'd say it was user error by him. The hvmaxx motor is for use with the hvmaxx esc, or a similar spec'd esc- mmpro for example which is rated for about 80-100amps. Making the buyer realise this will be fun though....