Learn English – What does “following by” mean

It's a flowery way of saying if. It basically means the same as:

If we cannot assist you now, we will keep your information on file for future possibilities.

It looks like he's saying he'll take what she says at face value because she's kind and honest. A lesser woman might tell a man she isn't romantically interested when she actually is (or vice versa, the quote doesn't quite disambiguate), but Elizabeth is "too generous" to do so.