Cat in heat, but can't let her outside?

My daughter's friend's cat is in heat. She's an indoor only cat and is nine months old. Obviously, the owner has not yet had her spayed so the cat is going nuts not being able to go out and mate. The owner feels bad about it but doesn't want to let her outside because there are some male cats that have already come around and the cat will get pregnant if they let her out. Is there anything they can do to make the kitty comfortable until this heat cycle passes? I suggested to just give her lots of attention and scratch her head & back and be patient. She's also releasing some liquid and I told them that there's probably nothing they can do for that. I did urge them to get the cat fixed as the heat cycle could come back within 2-3 weeks. Thanks in advance for your suggestions.