To think she could have sat in a normal chair?

Without knowing the full ins & outs of how your friends runs her house, you are being unreasonable for making presumptions. My dining table is vvv expensive, the fabric chairs were made to order, and whilst durable, I don’t allow my toddler to sit in them to eat her lunch.. she’s petite and still fits perfectly in her highchair, so while we’re at this young & messy stage, she eats there. My table, my chairs, my preference! We saved for a long time for the furniture and decor that we want. Is there a reason your child couldn’t sit on your lap while she ate? Surely it wouldn’t have taken her long to eat.

Why on earth does a 12 yo need a special (expensive) chair? Any child 4 or over can just sit in a regular dining chair...

In this situation I would assume if they still have a high chair out then they are still using the high chair. I would text the friend and ask if I need to bring one. If they said yes, we have a travel high chair which is very useful for visiting people without high chairs or locations with out high chairs. We used ours a lot in the summer as you can put it on the picnic bench next to you and baby can eat with you on days out.

I wouldn't do that with a 3.5 yo but adjust the chair as it should be but tbh it's none of our business. Your child would be fine on your lap.

I just bought a new tripp trapp type chair for my daughter and they recommended the high chair with harness setting until 36 months... Which I found was hilarious 😆, but maybe she was following the chair's instructions.

Anyway, I still think slightly YABU. Even though I would have let the chair for the younger one too, she shouldn't have to change the rules for guests. You should have checked with her or bring your chair.

You assumed, you were wrong. Next time check if it will be available maybe?

That's so weird. DD has been sitting in a normal seat with a booster since about 2ish and by 3.5 didn't need a booster either. It's also really selfish of her.

You have no idea why your friend needed her daughter to be in the high chair and you are unreasonable to try to control it and/or criticise her. If you needed a high chair you should have checked if there was one available before setting off and if there wasn't one available bring your own or make do as best you can without.

If a 3.5 yo sits on a Trip Trap it's not a high chair anymore but adjusted to the older child.

No, but you check if the restaurant has highchairs. Like you check if your friend has hers free.

They are chairs not high chairs and tbh hell to clean. Yabu To expect use of it. Why didnt you take your portable booster seat. I got one free for marketplace but have seen them for £5. Easy to carry and use.

Trip Trap adjusts to the age of the child. If a 3.5 yo chikd uses it it's no longer a high chair but a chair adjusted for an older child.

We used Trip Trap chairs to around 12 yo with our oldst children. It's quite common.
The youngst had a Nuna chair. Nina adjusted as well to size/age around 6 yo we passed it on to a baby in the family.

Weird yeah neither of mine were still in a high chair at 3.5

Not sure why you're getting these responses OP. If for whatever reason she still has the baby set on the Tripp Trapp - which is strange for a 3.5 year old - then I think most people would have offered it for the 9 month old to use. This is especially true if the three year old was asking for a grown up chair, so no battle involved. It sounds like she's pressuring her daughter to stay in a chair that's too small for her for some reason, or wants her to be seen as the 'baby', which is strange. It would have been helpful to be offered the chair so I see where you're coming from.

Too old for a high chair I had a booster seat that strapped onto a dining chair at that age.

Lot of fuss about nothing

I would never think of offering a Trip Trap adjusted to a 3.5 yo to a 9 month old.
It's not a high chair then in use for a +3 yo.
I would concider it too dangerous to use it for a baby without the baby set.

I don't know why you didn't just keep your baby on your lap. Doing things one handed with a baby on your hip is par for the course for most parents. Including eating.

Maybe she doesn't want her dd to think that eating anywhere other than in the high chair is an option.

@coxesorangepippin that reminds me of when my now ex SIL invited us for dinner. She did turkey dinosaurs for the DC. Her DS had to have one of each shape apparently. Which means my DC ended up with 1 dino each. Her son nibbled one, threw one on the floor. She also didn't cook for us adults. We went to McDonald's on the way home!

Your friends behaviour is odd to say the least. Unless her daughter is SEN I’m very surprised either would want her in a high chair still.

But your are being U for feeding baby in car seat instead of sat up on your lap as eating in a non upright position is a massive choking hazard for that age group.