What's the point of marriage if there's no intimacy?
Flip this on it's head for a minute:
What if his wife was the one trying to keep the spark alive and he just didn't want to know? He pushed her away all the time and made excuses every time she wanted some.... servicing, you know? She's going to start checking his communications; phone, emails and mutual friends in case he's cheating on her. She's going to lose her mind trying to work out if he's been unfaithful. Then let's say she LTB and it turns out he never cheated once, he was simply stringing her along the whole time, the divorce has been finalised and she's in her new house crying and feeling sorry for herself?
This all happened to my mum. My dad was the nicest man you'll ever hope to meet, he's now in the latter stages of dementia and hasn't had a clue who I am since before I got married over a decade ago. Rewind back to 1990 my dad was madly in love with my mum, he gave everything of him to her, but my mum decided she'd divorce my dad after pressure from her female friends (this is why I think divorce is a social contagion), I remember hearing my mum crying for days and asked several times out loud: "What the hell have I just done?!" She later went on dates and I once walked in on them both macking in the kitchen. He even gave her a hicky, 10 year old me was thoroughly grossed out and went back to my room. That one didn't work out for her as it turned out he didn't want to date a Jehovah's Witness 😂
No marriage is perfect. Mine certainly wasn't at first, but we worked hard, and I mean really, really hard and now after years of miscarriages and infertility, we're expecting our first baby together. He's walloping me as we speak.
Yes, no one is 'entitled to sex' as you put it, but it's a vital component of any lasting marriage, being there for each other, taking care of each other's needs, loving each other and being there for each other, emotionally and otherwise. Even if there's perhaps an 'anatomical mismatch' of sorts, like say he's a little big down and she's very small? If you know what I mean. All the stretching in the world isn't going to fix that, especially once it 'snaps back' and it gets painful again, the sex ends up requiring additional steps and it spoils the mood a bit. So change the approach and do other things, sex needn't be parts interlocking and if you want to make babies, there is always artificial insemination.
One thing that is important to note is that sex needn't be something that happens every night. That's exhausting. Every now and again is perfect and will make the experience that much more passionate and exhilarating for the both of them. Perhaps spice it up a bit with some dress-up; sexy French maid or nurse, or him leaving very little to the imagination etc Ann Summers has some really good stuff for this, by the way. Oh and if it's a very hot night in the middle of summer, try ice cubes down the back or the front. Get creative. It needn't be BDSM, I think a whip and ball-gag might be a little far, but make it interesting, you know? Who says this stuff is exclusive to younger couples?
I think this couple will be fine as long as he keeps fighting to keep her. She's probably in a state of mind where she's unwittingly locking him out. Counselling might also help.