Like Dave has said it is not your responsibility to ensure he has planning permission for any of his signage so there is nothing the council can do to you personally. I always used to make sure my customers were aware that for certain types of signage planning permission was required and I also used to try and steer them in the direction that the council favored. I also used to keep copies of the advertising regulations on the counter for customers to see and take away if they wanted.
Councils seem to interpret advertising regulations in different ways and what one council will allow another will not, I also think the size of the company makes a difference because large companies who employ a lot of people locally seem to get away with things that small businesses do not. If you think your council will not allow these signs and you have advised your customer of this then it is really on their head, but as Dave said make sure you get paid straight away before he is ordered to remove them.