Just take it outside.

Get a crate, put the dog on a new bathroom schedule until he becomes familiar with his environment. Make sure your friend goes outside with him and gives the dog a treat immediately after he goes, don't wait till he/she's inside to give the treat. Go to a pet store like Petsmart or Petco and get a bottle of Nature's Miracle as well (for when the dog has an accident inside), it eliminates the residual smell. Regular cleaners aren't going to cut it, it might get rid of the smell for you, but the dogs can still smell it, and they'll be more inclined to use the bathroom in the house more often. If you can't find that particular brand, check the ingredients and find anything with a bio-enzymatic formula: it'll eat organic materials like urine or feces, it's also good at getting up stains. Or just tell your friend to do all this. Don't get rid of the dog, it's not his/her fault, some dogs are just unsure about new environments and are trying to familiarize themselves with their new digs. Above that, your friend is going through a divorce, and getting rid of his dog is really messed up. I understand it's your house, but the solution to this is very easy. As far as the crate/bathroom schedule goes: Keep the dog in the crate, let him out in about an hour to go outside. If he/she doesn't go, put him/her back in the crate, and try again in 30 minutes. It shouldn't take long to get him/her back on track if he/she is already potty trained. If he/she has an accident in the house, don't yell, don't punish, just clean it up and put him/her back in the crate. If you start getting mad over it, the dog might confuse this with getting in trouble for using the bathroom in general, and it'll make things worse as they'll start holding it until they can't anymore, increasing the chances of indoor accidents. As Progo said, dogs typically won't use the bathroom in their crate unless they have a legitimate accident. Despite the fact that dogs generally seem fine with filth, they actually like to live in clean conditions.

Keep it outside?

Let the doge stay, help your friend get back on his feet and don't take away what he is quite likely clinging to for support. (His dog) There are a few ways to deal with the mess and people have suggested those, but kudos for looking out for your buddy in the first place.