A professional upholstery cleaner might be able to remove both the stain and the smell. If a professional cleans the couch and any smell remains behind, you can remove the smell with Lemon Scented Pine Sol. I had a problem with cat urine smells when I babysat my father's cat for 3 months.
I tried Vinegar, Baking Soda, Enzymes, and Febreze. None of these worked for me. What finally eliminated stench of pee was Lemon Scented Pine Sol.
Nothing else even dented that smell. If Lemon Pine Sol can eliminate the stench of cat pee, dog urine is a piece of cake. I would start with a professional cleaning, as mentioned.
After the cleaning, pour undiluted Lemon Scented Pine Sol into a spritzer bottle and spray the couch with this. Allow to dry. Small warning: the scent of the Lemon Pine Sol will be absolutely overwhelming for a while (it actually made me woozy), but the smell dissipates.
And once it does the urine smell will be gone -- not covered up, but gone. You might have to repeat a couple of times, but this works. Make sure you use the Lemon Scented Pine Sol, not the regular scent.
You have the roommate buy a new couch. There is no getting the smell out.
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