I just can't believe some people. With DS1, before any details were arranged, bride, bride's mum and I (groom's mum) sat down together and shared out the duties. We said we'd just keep all the receipts and divvie everything between the three of us (bride and groom, her parents and us). Her parents paid for the dress, the rest we split three ways.
DS2 didn't want anyone there at all. In the end 13 of us turned up. No rings, no flowers, no dresses (DS2 was barefoot as usual). Nice meal afterwards, the bride's father picked up the tab.
OP, I wouldn't know what to do in your situation. I would definitely tell your son your thoughts and hope the wedding gets called off. If your Dil-to-be is a nice girl really, perhaps she is being pushed by her family? No hope of confronting the mother, is there?