Help!

Went to a neighborhood event Sunday and learned, to my horror, that my
male, spayed, 4-5 year old cat has been spraying in 2 houses.

The 2 neighbors didn't bring up the matter; it just developed in
conversation.  I feel absolutely SICK!  Wish they had TOLD me!

Neighbor #1 says that my cat is over there alll the time!  He sneaks
in whenever she leaves the door open and sometimes gets locked in when
she goes out, not realizing he's inside. He has pee'd on her couch and
sprayed on her drapes.

Long ago, she mentioned that my cat visits often, and gets on her bed
with her cat.  I was horrified, but she said they seem to enjoy each
other's company.  So there was no cause for alarm.

Now, it looks like my cat has taken advantage of her
welcoming/passive/whatever attitude to start camping out there.
She says she likes him, but major reprogramming is needed here,
seems to me.

I suggested she turn the hose on him.  It's only water; unpleasant but
not harmful.  After a few such "welcomes" he might get the idea.  But
she's in a 2nd floor apt; can't access hose.  She suggested spray
bottle.  I concurred, urging good strong spray.

What to add to the water?  I have heard cats don't like citrus, esp.
oranges.  I'm thinking of grinding up orange peels, straining, and
giving her  the concentrate to add to spray bottle.  What else --
repellent but not harmful?

Yes, I will ask the vet why cat has started spraying.  I can hardly
afford another visit, when he just had his annual physical, but...

Thanks for any comments; ideas...

Desperate