Simple RAW 3

[Hyde]

Q382 Is there any way for a Druid to teach their Rituals to a Tomelock?

A382

Book of Ancient Secrets Invocation: "On your adventures, you can add other ritual Spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell’s level is equal to or less than half your warlock level (rounded up) and if you can spare the time to transcribe the spell. For each level of the spell, the transcription process takes 2 hours and costs 50 gp for the rare inks needed to inscribe it."

It's more or less up to your DM what "finding" a spell means, but personally, I'd allow it.