Tom cats marking

Looking for some advice for a customer of mine!

She has a female (spayed) cat, and during the day when the lady is out at work, a tom cat comes into her house (through the cat flap) and sprays everywhere! Her house smells so bad when she gets home from work!
He has ruined curtains, carpet etc.

He's also doing the same to 2 of her neighbours. They don't know who's the cat is.

Is there anything that can be put down/used to try and keep him away. Everything we could think of would keep her own cat away too, so we've run out of ideas!
He never comes when her or her husband is at home, so they can't scare him off!

I told her to clean everything with bio washing powder, to ensure the smell is out of her belongings (she's having to wash her carpet weekly!

She can't put a collar on her cat for an electronic/magnetic cat flap, as her cat once wore a collar and dislocated it's shoulder trying to get the collar off, so is now terrified of them.

So cat owners....any ideas?!