Is it unreasonable to be frustrated by a guest who says they eat anything when they actually don’t?

I think the responses are pretty harsh. Sounds like MIL is trying to not make a fuss by saying she'll eat anything (ok, she doesn't, but who does? Roast tarantula anyone? Crickets on toast?) then you're serving up chickpeas... I bet a sizeable number of people given the choice between chickpeas or chicken and two veg would go the latter. Cut her some slack.

Gen Z? Taco bell? We were eating tacos in the 90s in a working class household

Yes, but the OP asked her MIL about food. That was MIL's opportunity to say 'I always have a boiled egg, toast and marmalade for breakfast' or whatever. If you say 'I'll eat anything' and then balk at granola and yoghourt you only have yourself to blame! To rub salt in the wound, the MIL then went to M&S with the OP and for whatever reason refused to suggest anything she'd like to eat. Cutting off her nose to spite her face.

Salad cream is a marker of working classes, I know it because it is what my grandparents preferred. I think that you are pushing the wrong class of food on her - my grandparents would not entertain chickpeas, or tacos. Ethnic foods are the preserve, I think, of the colonial upper middle classes or ethnic communities for that age category - I remember as a child going to a friend's house in the early 1980s and trying for the first time chickpeas in a curry so hot, that there was an obligatory sliced banana on a bread plate for everyone, but her grandparents were born in India and were used to it, their parents were in the Colonial service. I think that it is only young-ish people who embrace all that these days.

Gammon with pineapple was a standard item on Berni Inn menus and the like back in the 1970s. I used to visit an old lady (possibly born in the 1890s) who thought meat and fruit together was an abomination, and specifically mentioned gammon and pineapple. I suspect she would have been perfectly happy to eat chutney or Branston pickle, which is full of fruit and eaten with savouries.

A tagine might be bog standard for you, not necessarily for some others.

Gen Z? Taco bell? We were eating tacos in the 90s in a working class household

She obviously assumed everyone eats beige food and doesn’t consider that the healthy options you were offering her actually qualify as food!

How old is your MIL ? Some previous posts are terribly ageist . I'm in my 60s and yes I grew up with Vesta dried curries and Bernie steakhouse regarded as exotic but regularly cook and eat a wide variety of ingredients (including fajitas but not tacos).

This idea that anyone over 60 only wants to eat "traditional" food is nonsense.The huge increase in variety of food consumed in the UK was driven by "old" people back in the late 70's and 80's.

I don’t mind chickpeas in moderation, eg as a side dish with a restaurant curry, but I think it’s being deliberately disingenuous to offer them as a main meal on a guest’s first night.
I also don’t believe that this makes MIL a “meat and two veg person”… I bet she eats other food. OP’s menu is particularly Gen Z ( Taco Bell anyone)?

And doesn’t the original Coronation Chicken recipe contain fruit?

yeah and ox tongue and ox tail as well as other hideous animal body parts

I’m surprised by people saying fruit in savoury food is modern. Most older people will have heard of Duck à la Orange, even if they hadn’t tried it and I can distinctly remember having curry containing apple and sultanas in the 1970s.

Yes, flicking through an old cookery book, the victorians were into dried fruit and spices (but not chilli) in all kinds of dishes.

I’m surprised by people saying fruit in savoury food is modern. Most older people will have heard of Duck à la Orange, even if they hadn’t tried it and I can distinctly remember having curry containing apple and sultanas in the 1970s.

I think the responses are pretty harsh. Sounds like MIL is trying to not make a fuss by saying she'll eat anything (ok, she doesn't, but who does? Roast tarantula anyone? Crickets on toast?) then you're serving up chickpeas... I bet a sizeable number of people given the choice between chickpeas or chicken and two veg would go the latter. Cut her some slack.

It's because she cannot imagine a taco, basically - she does eat anything in the very limited range of things she encounters 😂

Let her son deal with her.
I

boomer here, no it isn't. I have always got mayo, salad cream (call it remoulade to be posh) thousand island and tartare on hand.

I have noticed this too. Marks still seem to have stuck to their curry gradings but loads of their ready meal stuff is now much hotter than it used to be and, as you say, adding chilli to dishes that traditionally don't contain it. They also seem to have this "thing" about putting fashionable ingredients in everything. I am looking at YOU chorizo and nduja.