That said, I think the fear of no win, no fee legal firms is a little overblown. You can't get blood out of a turnip and if he's not making money there's no reason any law firm would be interested in suing him.
I'm honestly wondering what the law previously said regarding data protection. The Dutch WBP from 2001 already covers everything that he would have to do under GDPR given this website, so unless the UK has some very weird laws (or unless we're weird), nothing would change. Perhaps an extra tickbox on signing up that says "yeah yeah I'm really very aware that my data is shared with third parties".
Most likely, this is a good excuse to go "I refuse to read the long legalese [even if it's 95% the same as before] and I'm just going to quit this loss-turning website without the community turning sour on me because I have a good excuse".
Generally speaking, I think most sites will require some additional development work to update their UI and to store the data. If you have been compliant with the previous laws then it should be no problem. One wrinkle is that in the previous laws there was no way for your customer to find out if you were following the law or not (without suing you). With GDPR they have a right to both be notified what you plan to do and the right to request evidence that you are following that plan. There will be many companies that will have to alter their processes significantly to account for this.
In the businesses I'm working with, I'm finding the biggest problems with GDPR aren't with GDPR - they're because the business wasn't compliant with the UK's Data Protection Act (1998). So the pain is the scramble to catch up.
I think you could ask that question about all the other formalities you have to engage in as well if you run a business? Managing his taxes probably takes more time than making that kind of business GDPR compliant