DSis has been with her DP for a couple of years. They have talked about marriage and agreed it's something they both want to do. She wants the big 'surprise' proposal though so as far as she's concerned, they're not engaged until she gets the proposal and the ring.
He told her last Xmas that he would propose before the autumn. Deadline came and went, no proposal. She called him on and it and now he says he will 'definitely' propose by March. He's adamant he does want to get married and from an outside perspective, he does seem to be crazy about her and seems like a decent bloke. She is worried he's getting cold feet about making such a 'grown-up' decision - she may be right. He's 42 by the way 
Added to this, she has very fixed expectations about the ring - she's expecting him to spend 3 months salary on a rock (based on what her mates have had for their engagement rings), he was expecting to spend nowhere near this. He works full-time and has some savings but is nowhere near loaded.
Is there anything I can do? Do I tell her she needs to manage her expectations regarding the size and cost of the ring she can expect? Or do I hold my tongue and go along with it? I really would like this to work out for both of them and I think they are a great couple in many ways but I'm worried that both of them are getting very stressed out by something which should be a happy experience