The employer has provided the PPE but he wants to supply his own more comfortable kit, what's wrong with that?
Because it is the statute law duty of the employer to provide suitable PPE, if it is not sufficiently comfortable then it is not suitable.
Also if he provides his own PPE and it fails, and he dies, do you really think that his employer is going to pay out to his family?
He was not using the PPE that the employer deemed suitable, he did not raise an issue of any kind with his employer he just used his own, which was not under the control of his employer, nor deemed suitable, thus he is on his own, and now his family will get a reduced pay out because, he spend his OWN money buying kit that his employer has a statute law duty to provide and which must be suitable and sufficient, and comfort is part and parcel of that requirement.
Now I don't know the OP from Adam, but, I would argue to the ends of the earth to protect him in his job, and if things go badly wrong, I would entrust that any advice I had given, even on some random internet forum, would protect the OP & his family AFARP, and ensure that they would get the best outcome.
Please consider your responses and understand the laws and the ramifications.