That was your indication they weren't likely to pay and to have dropped asking at that point. Keeping going at them will eventually make the person feel harassed even when they know darn well they should be paying you pay (that's not meant to be in their defense just that they can eventually claim you are going after them even when you have the right to get back the money they advised you they would pay back). It is different than an agreement between parties where you are loaning them money but even then eventually you'd have to stop asking and take it as the money is lost, it is sadly an understood thing about paying for things for others/loaning people money.
Truth be told I'm having a hard time reconciling you both having been friends with someone for two years, liking them and them seemingly doing an about face on behaviors over one dinner. Is it possible that there was things afoot before this? Random comments made either by you about your seeming ability to have less financial stress or by them about having more financial stress? Or random comments or behaviors that would lead you to believe these people aren't as warm and fuzzy towards you as you think they are?
I am with the others in that this friendship is probably fallen out but I also wonder how close of a friendship it was. I absolutely adore my friends but they also aren't the people (the ones we go out to places where this situation could arise) who would even suggest at having us pay and then they'd pay back given this upscale locale. The honest way would be to either cancel or select a different locale more suited to the budget of the evening; these supposed friends of yours come across as just wanting a high price meal free.
Speaking to the honest way I can't say I would just offer to pay for a friend's tab at an upscale restaurant with the excuse of a forgotten wallet (ETA: willing to bet they didn't forget their wallet, they either didn't want to pay so hoped you'd pay or they are extended beyond their means financially and hoped you'd pick up the tab without giving the appearance they are hard up for cash), then again separate checks are the norm around here and that differs regionally (DIS-style

) That's not to say no one has treated friends but it would be quite rare for your situation to even occur if I'm being honest with the people I'm around and if it did a polite but firm "I'm sorry I can't do that" or "would you like to go elsewhere" would be said, putting up a fuss about that would tell me all I need to know about the people and would affirm my choice in not acquiescing to them.