Needs his salary for the mortgage?
Bugger that!
She should get him out - now, before he's paid a penny towards it (which might give him some claim), and sue him for the cost of the wedding. And for maintenance for their child.
This hasn't happened in a heartbeat - he's known for a while but has either
a) got cold feet and been too cowardly to tell her, or
b) just got the go-ahead form some other woman he's had his eye on for a while.
"Needs his money for the mortgage" suggests that he intends to stop there as some sort of a lodger (or continue the way they were before, with sex on tap, so to speak) so that Claire can continue to do all of the hard work around HIS children, and look after him. She wants her head looking at if she accepts this.
TBH, I would suggest that she chucks him out - it's effectively her house (though she may have to sell it straight away she can claim her deposit before any "sharing out" takes place), cancels the wedding, gets any deposit money she can back from the venue, and if not braves it out. Tells the relatives to come and celebrate her lucky escape.
If that money can't be reclaimed, they might as well have a good night of it, even if she can't bring herself to join them. Explain what has happened, let them all see what a shit he is.
She is still in shock at the moment, poor woman, but if this is the sort of bloke he is then she is better off out of it sooner rather than later.
She is upset and humiliated. Turn that distress into rageful vengeance and get rid of him. Even if they stop together for the next 20 years (they won't) she will never be able to be sure of him. She shouldn't be prepared to be second-best in her marriage.