Flight canceled due to shortage of crew caused by bad weather

My brother was returning from Prague to Bristol (where he works) on October 5, 2017, but his flight (with easyJet) was cancelled due to bad weather. He was offered a flight the next day in the morning, but that flight was to London, so he had to pay for a train from London to Bristol. I advised him to send a compensation claim, and this is part of the answer he received from easyJet:

To further explain what happened on the day; on the 5th October there were storms and gusting winds across Northern Europe which caused substantial delays as air traffic control restrictions were put in place to deal with the situation. These restrictions had a knock on effect and intensified during the day as aircraft were waiting for take-off and landing slots. Ultimately the delays pushed our crew out of their maximum legal operating hours to complete your fight. There are strict industry wide rules on the number of hours our crew is allowed to work. To protect the safety of our customers and crew these cannot be exceeded so we had no alternative but to cancel your flight.

We do take reasonable measures to avoid delays and cancellations by having replacement crews and spare aircraft in our network. However on 5th October 2017 there were higher than expected levels of disruption across our network and therefore these replacement crews and spare aircraft had been deployed by the time of your flight.

So basically they refused to compensate him - he was only offered a refund for the train he had to take from London to Bristol (which is also written in the mail).

I know bad weather is one of the extraordinary circumstances outside of flight company control and therefore, under EU regulation 261/2004, my brother is not eligible for compensation.

The problem is, that although the weather was bad that day, I think it wasn't the immediate problem for this flight - even easyJet says in the email that the reason is a lack of replacement crew, due to earlier problems.

I also checked other flights from Prague to UK and they have departed without any problems - look for example at this one: https://jakubm.com/flights/324229230 - it's an easyJet flight from Prague to London, which departed at 23:03 (scheduled at 21:55). And this is my brother's flight - https://jakubm.com/flights/324220100 - it was scheduled at the same time, but was cancelled instead of being delayed, like the other one.

There are other flights that left earlier that day:

17:15 - https://jakubm.com/flights/324271710

17:00 - https://jakubm.com/flights/324208900

19:40 - https://jakubm.com/flights/324633710

So my question is, is he really not eligible for compensation, even though different flights from the Czech Republic to the UK from easyJet departed, but this one was cancelled due to shortage of crew, caused by bad weather that day?

Also, in the mail he received, they advised him that if wants to challenge their decision, he should use https://www.cedr.com/aviation/ which is their ADR supplier- however, they charge a fee of 25 pounds if the claim is 100% unsuccessful - does easyJet have the right to deny further discussion about this issue and redirect my brother to CEDR?