**Edited to add - my house is clean except for dog hair
( naturally ) no other pee spots anywhere else.

Often when a male dog urintes indoors at someone else's
home or his own he is trying to stake territory. This is
not a house breaking issue, it is a territorial behavior
issue. Let us assume he does not do this at his own home.

Where does he stand in the pecking order with the other
male dog? And even your female?

If he is the beta, he might be trying to let your female
know she is below him in the pecking order. If he is the
Alpha male he is trying to let your female know
that he is top dog. Keep in mind he does not know if he
is here permanetly or just visiting. In his mind the
pack is unstable. There is a new memeber and he wants her
to know where he stands.

In a wolf pack, the alpha wolf will usually be the dog
that marks territory. No other wolf would have the
gumption to do this unless they are going to challenge him.
They know where they need to eliminate and they do this
with respect. This dog does not know who is in charge and
may feel he needs to be.

From his perspective he is thinking... Who is top dog?
Who is the boss? Where do I fit in? I don't want to loose.
So lifting to mark an area is his way of saying I am part
of this space. He may be setting boundaries to prevent a
fight. He may not be a fighting guy but wants everyone to
know this is his space.

Some ideas that might help are having a crate in your home
for him to enter as soon as he arrives. After 1/2 hr let him
out of the crate but watch him closely as though he were
a tiny puppy. Keep him on a leash attached to his owner
or even you. Do not let him out of your site and if he
lifts a leg. Be ready to train! As soon as he lifts
a leg, shake a can with coin in it. Scream loudly. NO! And
rush him outside to do his duty. If you can even mop up
his pee with a paper towel and leave it outside where he
can smell where he needs to go.

Be loving, firm and consistent. He needs to be taught in a
loving way that he is not the alpha of the space. That
someone else is in charge.

He needs to earn your trust which means he either needs to
be closely leashed to his owner or you and slowly when he
can be trusted not to lift his leg he can roam free.
Never reprimand a dog for marking if you do not catch him
in the act. This will only confuse him.

I hope this helps.
~boxerpups