Frank Churchill is writing to an upright, sedately-married female mind when he uses the terms:

'I was fortunate enough to prevail, before we parted at Weymouth, and to induce the most upright female mind in creation to stoop in charity [...]' (50).

Even had Miss Fairfax only offered a half-hearted demurral before accepting, Frank would have implied a more principled response. (I do not say that Miss Fairfax agreed without hesitation to a secret engagement: Author Austen makes it very clear that Miss Fairfax is to be considered a well-behaved, elegant, principled, religious young lady. I simply want to point out that to me Frank Churchill's praise has an much weight behind it as, say, Caroline Bingley's censure of Mr. Wickham to Elizabeth at the Netherfield Ball.)