Since your bf already knows and acknowledges her odd behaviors, you're well ahead of the game, compared to if she had him snowed, which would be a much trickier situation, and probably grounds for just giving up... However, I wouldn't personally advise for getting in between your bf and his mom in any way, beyond just reasonable boundary-setting and whatever he's genuinely ready for .. can of worms. Just my opinion, among a reasonable array of different opinions.

As it is, she sounds like a very sad woman, with very little of her own inspiration in life that she spends her time and money trying to impress others with how much better she is by virtue of newer cars, newer phones etc .. I mean really, these are the priorities of a teenager. My own experience with deeply competitive people of a certain age is that it is covering up insecurities they would be loathe to unearth even in a therapeutic space, and that they will defend to the death their right to continue wearing that mask of protective steel. Perhaps hers could one day pry loose more easily; I pray for you this is the case.

However, in terms of working with or around her .. "features" .. is it possible you could draw her attention to the error of her ways in real time, such as, instead of offering an opinion on something that is sure to be shot down just by virtue of it being an opinion of yours, say something directly to her along the lines of "well I'm sure there's nothing I could add that you'd find valuable", in an only very slightly snarky way so that she gets the meaning but would still be forced to ask you for your opinion, on the very basis that she would indeed find it valuable? I mean, the only other possible response would be to agree or to agree through silence, and that would be impolite in just about any scene I can think of.

Just an idea I'm offering up; it may not work for your situation or for the particularities of her personality. Give or take.