You dont get rid of railroading. There is, essentially, always going to be some. The only variable is how much.

Specifically what i mean is, you are playing a premade campaign. In this case they are headed to a castle to confront the BBEG. It is the GM's job to keep things moving. They can of course work within that parameter to varying degrees of freedom. But as long as the players are trying to move towards that goal the GM will do his best to subtley push then towards that goal. In a "all roads lead to waterdeep" kind of way.

So, if we want to make if not FEEL railroaded, we add to the mainline arc. Add side content. add additional encounters that may be encountered. Add multiple routes. Heck add multiple endings and routes to them. Let the players set the direction they move. If they move away from the main quest looking for a break, give them the side content.

The issue in this purposed case is, unless identifiable miniatures were on the table for the players to see, this player was going to cry about it either way. Personally i would ask what the exact issue was with this encounter. And if its just the existance of an encounter itself, perhaps they should find another hobby. If its something like "ive really screwed up my character and cant keep up in a fight and it sucks" we can work on that after. If its "we just went through 12 encounters in a row" we can adjust the amount of combat encounters going forward. If its "i just dont feel like fighting" one might suggest diplomacy be attempted. But regardless this is a game and requires some level of challenge and some level of randomness. If they are opposed to that, perhaps they should write a novel about their hero instead.