They may be lovely people, but why on earth do they leave two dogs outside for hours on end, barking or not?
Is there a possibility you could get someone else to approach them to report the barking? I know how hard it is when you're trying to run a business - you don't want to risk alienating them (or their friends!).
I stayed at a guest house once where there were Newfies; they were much-loved, lived in several acres of land and spent most of their day outside while their owner worked alongside them...but they barked at everything which moved. Frankly, that would drive me crazy, but at least they had no close neighbours and we only heard them when we actually walked past.
I once had newish neighbours whose dog barked every time they left him - and since he had access to the garden, that's where he did his barking. I hate confrontation and didn't know them well, so one night when it had been particularly bad, I popped a note through their door, saying how upset he'd sounded and that I knew they wouldn't realise since he stopped the minute he heard their car come back - adding that I'd be happy to go over and let him out for them any time (meaning they could keep him indoors!). I never heard him bark at night again - they never mentioned it, but we're still on speaking terms.
I think it's downright anti-social of people to keep dogs outside and allow them to annoy other people all day long.