My roofer has removed a row of tiles.

All common tiles and slates are within certain gauge , so ideally you should have the same rows back on for different reasons , one is to have correct coverage and 2 so they line up with the other tiles , some larger tiles interlock so they need to be exactly the same to sit right

He probably opened up the gauge to avoid cutting the last course of tiles a bit lazy, but as long as it’s not past the maximum recommended gauge set by the manufacturer for the tile and pitch of your roof this is fine. Personally not how I would had done it myself as I care about the visual look as well as the roof working as intended but each to there own as long as it doesn’t leak and isn’t a mess it doesn’t matter. Have you spoken to your roofer and asked him why his done this? Might be a good idea he can put your mind to rest then I’m sure he won’t mind you asking I think he would be more bothered you writing up on the internet about his job. I certainly would anyway I would think it was rather cowardly of my client not to just ask me direct if your too scared or nervous to ask him them questions you shouldn’t had hired him in the first place and I wouldn’t one somebody to hire me if they felt nervous around me to ask questions as it just put awkward atmosphere on the job.

Whatever he has removed he should replace the same unless changing 100% of the roof and the roof structure because this will only cause problems in the near future I would highly recommend you get another Roofer to give you a second opinion hope this helps thanks

If you roofline ties into the neighbouring roof and the tiles are a like for like replacement, only newer then you should really follow the same gauge so the roof(s) remain symmetrical. Gauges can be calculated over the elevation span. If the tiles are different and bonding gutter is be installed then it should break your eye line and stand out as two roofs, do not so. important. It’s really down to the roofer to point out any issues and finishes with the customer and agree before installation.