OP: if you are still intent on getting through to her, you can point out that spending this new-found money on a purse instead of investment doesn't cost $900. Over the course of 20 years compounding at 7%, it will cost $3,482.72.
(I can tell you which one I'd rather have!)
That's a lot for a purse, but if this is a purse she will be using regularly for the next several years (or more), then I say more power to her. If it's a fashion statement that won't be in style next year much less in 5 years, then hopefully it will bring her A LOT of happiness in the mean time.
This sounds a lot like wimpy consumerist rationalization to me. If high-end material possessions really do make you happy, I would recommend some introspection and really think deeply about what happiness is. And also, read a few more MMM articles.
For comparison, I won $1500 once and used it to buy a really nice black leather Coach briefcase. I still use it 20+ years later, and it still looks great.
It looks great because it was (apparently) well made and you took care of it, not because you paid $1500 for it. And I can tell you with absolute certainty that not all well-made briefcases cost $1500.
So, basically, very high quality leather goods can be worth the price, and she probably does need to celebrate her new job somehow. If it was a laptop computer you'd probably not throw stones, and the purse will last longer. I'm sure now she will buckle down and put money into savings, right?
A laptop will not last as long as a purse, but it has much more overall utility and definitely did
cost somewhere in the neighborhood of its asking price to make. The purse didn't. And
I would still complain if she spent $900 on a brand-new laptop if her trade is not in I.T. because good middle-of-the-road laptops go for about $600 max.