Tiles breaking when cut: Reasonable excuse?

Sorry to say there is NO excuse for not being able to cut a tile because its to small and breaks.
no.1 if you set out your wall and your cuts before hand you wont get any very small cuts.
no.2 you can cut tiles down to 4-5mm thick if you know what your doing and have the right tools.

Measure up a tile, 2 inches won't break on a tile cutter so cut carefully with an angle grinder and with some TLC ensure it looks finished, it'll grout up fine with silicone beading to finish.

There’s always a way around it cutting a small bit tile. Tiles itself are quite delicate and can break very easy when cutting a fine piece due to it not having much strength in it. The best way is to it is to use a manual tile cutter and score where it needs to be cut then use an angle grinder to cut out the section and if it breaks it will break on where the line has been scored rather than the small fine piece that is needed

This is a prime example of poor setting out before tiling... If the tiler had set out the job properly then he would have checked for any small cuts and adjusted for this. This is the basics of tiling.... setting out!!!

What I would do is take a big breath and resign to the fact that I need to go and get the wet cutter.
Except that maybe the time this will take is not considered by the tiler to be worthy of the price quoted?

Sounds like bad workmanship I am afraid. There is no excuse for not making small cuts or worked out at the beginning where the tiles would of sat so that you would not have required a small piece. As you mentioned the grout does not look good, I do not think in terms of damp will be a problem but may need to keep an eye on it, Cheers Tim

there are different grades of tiles some are stronger then others so the weaker or thin tiles are more likely to break when cutting them

Your Tiler is 'Not a tiler' if he cannot cut a 2inch slither ? its not your fault, furthermore to think that filing gap that size with grout is acceptable is ridiculous,obviously poorly laid out to start with if room has slithers around.
a decent wet saw or angle grinder with a decent blade can cut 5mm slithers if needed ,but really if more attention to layout was done this would not be a issue,and you shouldn't have slithers in room. I feel for you, and dread to think what else is wrong based on above comments.

I would of used a wet tile saw to get a small but perfectly cut tile piece to fit in the gap. It stop the tile from becoming damaged or even break.

Your tiler should never end up with a tile of 2inchs. This is because the job as not been set out / laid out properly. (though it can happen in some rare cases, so I won't be to critical as i can't see the layout of all the tiles) I could be wrong, but at a guess, as you mentioned a window, he may of had a good finish on your window sill, placed one tile there, and was then left with approx 2inchs at the back (that can happen alot), towards the window. However, I have to wonder why a tile of any type can't be cut at 2inchs, as it's very do-able, very. There's no tile that I know of that can't be cut 2inchs in width. (my strongest guess would be, he's using the wrong cutting tool to cut that 2inch tile) Bottom line, a 2inch grout line is an absolute no no. 2inch = 50mm. Most grouts are designed/made to do lines of up to 20/25mm max / less than 1nch. Read the guidelines on packet of grout.