Acclimating indoor cat to outdoor cats

Hi, I've very recently moved from a semi-rural town to a city. Our cat has had the luxury of never really being exposed to other cats as we were in a second floor, center apartment. While some strays did roam the apartment complex, they never venture close to our apartment. Now we're on the on the bottom floor of a multi-unit house. Our landlord advised us prior to moving in that there is a catch and release older male stray that came with the lot when she bought it. He's very, very friendly and well socialized. He's completely unbothered by the presence of other cats and presents as non-territorial. We've taken to feeding him since our landlord who normally leaves food out for him has been in and out of town a lot. That said he's gotten very, very comfortable with us a frequently lingers around our front door which is completely windowed so our indoor cat can see him when he's nearby which she doesn't like at all. She shows some pretty significant signs of aggression and is highly territorial as we've recently learned.

I was wondering if anyone has any advice about the situation. It seems this outdoor cat is really stressing our girl out. She even recently had to be treated for cystitis due to crysal formation in her bladder. We're maybe attirubting it o the stress of having her territory encroached upon. We've been hoping that they'd get used to each other but it has been a little over a month now and she's still not adjusted to his presence even if it isn't direct contact. We've considered a pheromone diffuser, but it seems costly (but maybe the only real long term solution?).

tl;dr

Our cat isn't very happy about the stray (possibly strays) cat that roams around the property we've just moved into. Thoughts or advice on helping her adjust?