It's basically just the fallout of what she found from information rolls; Foster is/has come upon Carly when she finds 'Shadows'- following extra panic resulting from that, she would've gone to question RM about soul purge worries, etc etc...

It's largely offscreen talk by that point concerning the shadow book, the shadows in general, etc etc, but it leads to RM suggesting she ask Childermass about typical Shadowmagic rather than focusing on This Book (Or in RM's words, ask Childermass to teach 'If he can be bothered'. Thanks RM).

That in turn is partly because whether or not this book can be successfully translated in the future AT ALL is entirely up in the air; Quix confirmed that it might not be able to happen due to what shadowspeak is-'calling the shadows out by their name', in her words. Advanced stuff basically-RM herself doesn't speak it.

And then Option B on the book is literally testing it without understanding, but that's. Risky with or without shadow magic as it is. I digress-

I don't actually have anything specific in mind for a technique however-partly because neither would Carly, and more importantly that would still depend on what-if anything-Childermass would be willing to teach in the first place.

Presumably however it would be a relatively mundane spell.