I've never trained a dog but would agree with woofgang where possible.
It's really unpleasant to come out or home to find a dog has fouled on your property. I'm not sure where it's coming from but I've had a few instances lately of it on my lawn and, once, on the paving in front of my wheelie bin, being disabled and it being a challenge enough to get it out without trying not to wheel it through dog mess or try and negotiate round it on my own property, it's something I could do without.
I'm guessing it is dogs on the loose as I live near a large park and have seen a good few of them though a I know a couple who used to live down the road used to let their two out for a late night wander and ignore them doing their business on my front lawn.
Sorry for the rant! If there is the chance of an unplanned incident while out then could you carry a large flask of very hot water to at least wash it into a drain or something or offer to do the same if they can provide some if more is needed, at least show willing?