As a houseguest, you are showing appreciation by taking your hosts to an expensive restaurant. A friend of theirs joins the party. Do you also pay for the friend?
First off, I’d think the friend was pretty damned rude for butting in and no, I wouldn’t want to pay for that meal. Being who I am, though, I’d probably just cringe inwardly and do it although I’d totally hate myself for not being assertive and speaking up about it. Ideally I’d tell the person, “Yes, we’re here because I’m treating them to a nice meal as a thank-you for putting up with me. I hope you’re okay with paying for your own as I didn’t plan for anyone else to be joining us.”
This puts me in mind of the whole idea of splitting the cheque at a restaurant. I hate that idea. If I order a $10 entree and the others order $18-$25 main dishes and drinks/desserts besides, why should I be expected to pay for more than I ordered? I think that’s a shitty concept and I’d like to throw the most expensive pie I can find in the face of the one who suggested it and make that person pay for the pie.
That’s all I got.