It's possible that the owner didn't so much lie as assume. Many dogs behave very differently in their own home with their own family than they do with strangers in a new location. When I adopted my older dog, the shelter workers told me she was shy. I have never seen this dog be shy. She crawls into the lap of anyone who will let her. However, other people have told me that when I'm not there she will not approach strangers, and may even avoid contact with people she knows well. But if I hadn't left her with others, I'd never know. So this dog, who is calm and well behaved at home may be showing a completely different side of herself than her owners ever see, and if they haven't boarded her much, they may have no idea.
Additionally, dogs don't generalize skills very well. So she may be flawlessly house trained... at home. Just like you have to train your dog to 'sit' in many locations before they learn that sit means sit, always and everywhere, in order for a dog to be truly house trained, they must have the opportunity to practice the skill in a variety of locations (and with a variety of floor types!). Some dogs generalize faster than others, but if this dog hasn't stayed in many new locations (especially without the owners) they may truly believe her to be house trained. If they have wood floors and you have carpet/rugs, she may not recognize that fuzzy terrain isn't an acceptable place to go, or vice versa. Go back to potty training 101, and keep her confined or tethered to you whenever you're home, and quickly take her outside whenever she looks like she needs to go. Reward handsomely for going in the right place, and clean all mistakes immediately and thoroughly with an enzymatic cleaner so she can't smell it.
It sounds like your guest may be exhibiting a combination of poor generalization and anxiety. Dogs will often hold their bowels and bladder when stressed. Giving her plenty of physical exercise and mental exercises, like working on training or puzzle toys with delicious treats can help her calm down, but sometimes the only cure is time. In my experience, most anxiety-related behaviors resolve in 1-2 days on an initial visit. For the potty problem, you can remove her access to water a while before bed time so she doesn't have to get up at night to go, and confine her to a smaller, safe location. If she's kennel trained, great. If not, the bathroom with a potty pad and some bedding will work. The smaller the space, the less likely she is to potty inside. If you can, put a radio or something in there to make some noise - she may be hyper-vigilant to new sights, sounds, and smells in your house which can fuel her anxiety, and ... (more)