Not always true about the fee - it correlates to the variables of the booking, so those determine if a fee is required or not.
There's as much chance of there being a fee, as there not being a fee, without giving a specific example. The same is true for using a credit card, so no advantage is identified in your post.
The only "variable" which card fees "correlate" with is how much the airline can get away with. They have next to nothing in common with the actual costs incurred in processing.
Out of the twenty or so airlines I've flown with, not one had a higher fee for debit cards than for credit cards. Typically you have much less in the way of associated purchase protection, however, which is why most people still use credit cards.
Swiss charge an extra £4.50 for booking with a Credit Card from the UK, but not for a Debit Card. This is a small gharge to pay as you get several weeks before you have to pay, and get the extra protection that you don't get with a Debit Card.
One of the links that I posted earlier shows that booking with a credit card means that you can reclaim the full cost of a holiday, for instance, if a flight is cancelled.
Depends what you call cheaper. By using my credit card I have automatic travel insurance covering cancellation and consumer protection, something I wont get with a debit card. I have also had to use a couple of times so it pays for itself without it I would have lost the full ticket price with it I got a full,refund no questions asked.
Your friend is facing a problem that is not entirely new. For several years now those who manage to buy things online have been able to find cheaper deals for a lot of things. But to take advantage of those deals they have to be able to use the payment options offered by the seller. And in the case of Easyjet that leaves you with few to no free options to pay...one of the many reasons I avoid them if at all possible.
But your friend will come up against this problem again and again and should get a credit card. Or else your friend should get ready to pay more for the flight and use a debit card/transfer to pay for it.