Nothing wrong with the width as such. If the owner has a (possibly older) laptop with a low resolution screen, the site will look wider. You can see the situation somewhere like:
You should be more concerned with ensuring the site works for users than the owner. Good that you are using a basically responsive layout but core elements such as the contact form, page headings, navigation and product images do not work well on small devices. That site will see a fair amount of mobile use if ever getting any traffic.
Not trying to be too critical, I'm sure you worked hard and the site is pleasant but did note the owner said:
"I realise there are so many opportunities out there particularly via the web"
Not for that website at the moment. You need to get the site working for all/most users, sort out how people are going to buy anything (few will email ad hoc) plus how you will get people to the site.
Search engines aren't the only route but can help. At present they will regard that site as confusing, not even basics such as titles, descriptions, alt tags are suitable. You also need to set up core technical optimisation e.g. stop the site being available (and indexed) both with and without www:
So tell the owner not to worry about the resolution, just focus on what they believe the site is for and making that happen. Not beyond anyone and not magic, lots of research/online reading, hard work, focusing on users. If they stick at it, given a year or so the site could be of value to them. Purely from the search perspective, these offer a start: