Speak up or refuse job because of Welfare/health concern with request?

I think your offer to do two 15 minute visits and explain the reason why is perfect. After the client responds ( hopefully gratefully accepts), you can decide to book it or not. You may choose to ask if anyone else would be checking on the dogs because if that person is providing bathroom relief that may explain why the owner only wants to book 1 x/day. It isn't healthy to hold it for 24 hours and can lead to medical issues, such as UTI and more. You're perfectly justified to not want to be responsible for their well being under those circumstances.

In my area, the norm for most dogs who only eliminate when walking outside (no indoor pee pad sheets or fancier grass relief station) is 3 or 4 times daily. For pups who have a designated relief area indoors, it may be 1 or 2 walks daily (only less if they have private attached yard to use).

Wow that’s crazy! This sounds like a situation of someone who shouldn’t be owning a dog. I think we’ve all worked with clients like that here so it’s relatable. In my experience, everyone knows better then I do about their dogs (two finger bunny ears). As hard as it may seem you can’t get emotional about this sort of thing. You are hired to stop by for 30 minutes once a day. They clearly know what’s best for the dog (again bunny ears). It’s like telling another parent how they should be raising their children, you definitely don’t want to be playing with fire like that. It’s not worth the additional stress that may come with it. All you can do is verify with them (which you did) & make sure that that dog has the best 30 minutes of his/her life and is taken care of! If done correctly, you covered your liability, the owners know exactly what you did so there’s no confusion, and your conscious is in a better state knowing that you gave that poor dog the best 30 minutes of his/her life! I love dogs just as much as anyone on this platform does but I’d caution making “suggestions” to anyone else about what’s best for their dogs. Good luck!