Try Nice. It's farther away, but it's right on the train line. There are some moderately priced hotels near the main Nice train station (Nice Ville). Some of the blocks in that area look a bit scruffy, but I (then-66-year-old female) wasn't uncomfortable in the area; I stayed on the south side of the station (the front). He can try booking.com to identify places that seem to be affordable and have vacancies, then contact them directly to see if he can get a better rate than he sees on booking.com.
The train from Nice to Cannes takes 33-41 minutes and the regional trains cost about 8 euros each way. The trip from Antibes is only 3.30 euros, so he'd need to weigh the additional transportation cost against the (presumed) lower lodging rate.
A potential issue is that it looks as if the last train departs Cannes at 10:03 PM (on weekdays), and it's a TGV that probably costs quite a bit more than 8 euros without being significantly faster. (I got this info from the Deutsche Bahn website because SNCF wouldn't cooperate.) The last regional train departs Cannes at 9:39 PM. That seems too early for night showings at the festival, but that would be an issue even staying as close as Antibes.
Edited to add: I see some dorm beds available in Nice--even for the entire period of the festival. (What are his arrival and departure dates?) With the longer-than-30-minute train ride from Cannes, I think he'd want to be careful about the location to be sure he doesn't have a long walk or bus ride between the train station and the lodging place.
The Hotel Ozz very near the train station has one bed in a dorm available right now on booking.com. He could make a reservation with free-cancellation privileges and keep looking.
Nice has a second train station, Nice-St-Augustine, that's a bit closer to Cannes. There might be lodging options in that area, but I'm not at all familiar with it, and I don't know that all the regional trains stop there. That late TGV I mentioned above almost certainly does not.
