Is my friend’s boyfriend real?

I’ve (F36) known my friend “Lynn” (48) for almost 5 years now. We met through a local meetup when lockdowns started to subside. She’s my concert and coffee buddy, we share a love for the same types of music and a coffee.

Lynn has apparently been with her boyfriend “Leon” (M50?) for nearly 15 years. He is the love of her life and has taken such good care of Lynn over the years. He cooks, he cleans, he cares about her feelings, he’s good in bed.

But I’ve never actually met Leon. Never seen him, or spoke to him. Nobody we mutually know has. According to Lynn, he wants to meet me and have dinner sometime. But dinner plans have never actually happened, even when I’ve tried to set a time and place. All I’ve seen are some photos of a guy who she says is Leon.

I’ve also never been inside Lynn’s apartment. I’ve picked her up in the parking lot, but have never been invited inside. Generally, she has either come over to my place or we meet up in public.

She insists their apartment is too messy and also, Leon is supposedly terminally ill. He’s a recluse who works part-time from home and was supposed to pass away about 7-8 years ago. But he hasn’t yet. Therefore, he’s afraid of venturing out of the house or meeting people, catching a cold, and dying. So he’s just hanging on for as long as he can in their messy apartment.

Now that nearly 5 years have gone by, I can’t help but think something smells bad about this situation. My current hypotheses are:
1. Lynn dearly loves Leon is and trying to hold onto him for as long as possible.
2. Leon is very mentally ill. Lynn is covering for him for some reason instead getting him therapy. Or Leon is stringing her along with how sick he is and living off of her.
3. Leon doesn’t exist. Lynn is very mentally ill and has done a relatively good job hiding it for 5 years.

So how do I handle this? Do I try to meet Leon to get more information? Do I assume my friend herself is unwell and try to get her some help? Do I just stay out of it and take it her word for everything? End the friendship? I find myself avoiding her because I feel so conflicted.

TL;DR: I’m not sure if my friend’s long-term boyfriend is real or not. Her stories and lack of me meeting him reek of him living in Canada and my friend living in La-La-Land.