Bathroom underfloor heating - floor tiles moving and grout coming away

Hi Ann I'm sorry to say it but it sounds like he's made a real botch of the job! If there was that much movement on the floor then he shouldn't have even laid a tile till the problem was rectified. He should have prepared the floorboards or caberdeck flooring 1st ensuring as much movement had been eliminated as possible, by either screwing to the joists or even adding timber noggins between joists for strength, then he should have laid 9 or 12mm ply and screwed this every 150mm. If he had done all of the above then I'm sure you wouldn't be having these problems, did he use the correct adhesive and grout? Should have been flexible for both, this allows a small bit of movement without any issues. Don't let him near the floor with expandable foam or any silicone this definitely won't work, I'm shocked he's even suggested this. The only way I'm afraid to rectify this is to remove the tiles, which will probably damage your underfloor heating and start again, hopefully you haven't paid him yet and get him to do it properly or someone who can. He should be reimbursing you for your under floor heating and tiles if they can't be used again Sorry it's not a more positive answer but I do all the above and never have issues Danny