I definitely would not buy the jewelry unless you genuinely want it. It doesn't sound like that is the case. I would personally not buy and not say anything, but that is me. If pressed, I would say that I am not in a good financial situation to make a purchase. If pressured, and if I felt the person in question could handle some honesty (for some, it just creates denial or hostility), then I would say what was on my mind, but I do not generally find people open to listening to such things.
I believe the only two times I spent $600 on something other than rent was a typewriter my husband gave me for an anniversary present (we pool our money) which helps me write the books I'm working on, and, if you added it all up, attempting to have an eye surgery done on my rabbit, though several dishonest vets made off with that. I try never to dictate to others what they should do in life, but I personally would not be sending money their way either.