Coming from someone who worked in banking before, I think it's extremely unlikely that Morgan Stanley would risk its reputation, which it has built over DECADES, to do anything illegal in this case - many people are interested in shorting many stocks, it's a large market out there, what is the ROI of focusing exclusively on Lyft when it could potentially be illegal? Also, Morgan Stanley is a market-maker, it is the counter party to literally trillions of dollars of transactions, many of which may seem to have various intents when taken out of context, but mostly it is merely catering to market interest, and it is generally foolish to read into market-making transactions without enough context. (Not to say that financial institutions never do anything sketchy, but I'm just saying it's very hard to separate signal from noise here, while it's very easy for people who are biased to see what they want to see).
I've read a few articles on this, the original one from New York Post (https://nypost.com/2019/04/05/lyft-threatens-morgan-stanley-with-legal-action-over-ipo-claims/) includes an actual pre-IPO investor claiming that they did in fact do this through Morgan. They even outlined a scheme where they bothered to create a special acquisition vehicle through which individual investors can short shares. I'm surprised that what is not being called out in this story is that there was so much interest from Lyft's pre-IPO investors to bet against the stock. That's not a vote of confidence.