You can attempt helping her, but a marriage is complicated. And you don’t want to get too involved to the point that it backfires on you and gets you in some sort of trouble.
See, it wouldn’t have made a difference if your crush had married you or some other girls. No matter who he married…his mom would have disliked her anyway. Some MILS are like that. They’re so picky about this girl and that girl…and they’re so adamant about their sons only marrying their choice. But the sad reality is that some MILS will never be happy with any girl…not even one they’ve picked out themselves. They just want someone passive, who won’t make them feel insecure, or threaten their authority, and will do their bidding as a servant does. Where do you live by the way? In the UK? Heard several such stories taking place there.