A couple of thoughts for you:
1. Spam Rats is a non-entity and anyone blocking email solely on Spam
Rats' information is a fool. You can't be responsible for what every
damn fool does on the Internet, so if the problem is that the customer
sending small amounts of mail has run afoul of a single fool, you
should consider limiting your efforts to helping the customer get in
touch with that fool.
2. If the customer sending small amounts of email decides that
blocking of multiple mail destinations is because of Spam Rats,
they're almost certainly mistaken. Not about being blocked, but about
the cause. Find out what's really going on.
3. If the customer is sending mail without a valid PTR record then
they're probably on lists similar to rfc-ignorant as well. Check in to
it. And help them fix the PTR record.
4. If the customer is sending enough email to find multiple folks
relying on Spam Rats' information and they're doing it without having
asked for valid PTR records, that's enough of a red flag that it's
time for you to scrutinize just what email your customer is sending.