1st train (except Sat and Sun) leaves Cannes at 05.16 arrives Nice St Augustin at 05.48. From there it's a 5 to 10 minute walk to the shuttle bus stop for terminal 2 at airport where Easyjet flights depart ( so a little too tight IMO)
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If their travel is early morning on a Friday, Saturday or Sunday, the last nightbus N200 from Cannes leaves at 02.30 and drops off at Terminal 1 at the airport at 03.46, from where they could take a free shuttle to Terminal 2
Alternative is to, book an airport hotel for the night which should work out cheaper than a taxi fare, especially if they take the train from Cannes to Nice St Augustin the night before rather than the 210 airport express coach.
( this would save around 20 euros based on a return coach fare )
The Campanile and Etap are usually least expensive hotels and they can walk to the airport from there.if necessary