Paul,
Congrats on getting your first customer
The light-looking bit when wetted
can be a sign that it's viscose.
I almost bought a suite a few months ago, I spotted it was viscose (i normally hwe if tests are OK) but it had a kind of fleck pattern in it. Where I did my test clean with HWE it took all the definition out of the pattern, it just "dissolved" the flecked appearance into a plain pattern and it lightened the whole area. Out came the dry clean!
The other way fabrics can look lighter when wetted is if they're treated with a silicone-based protector. Normally though, that'll be a patchy or streaky appearance when a panel is wetted all over.
Book it in, make sure you have a hairdryer with you and do a hidden test area, ideally the bench under the cushions if it's upholstered all the way over. Dry it off with towels and hairdryer FULLY before making any decisions. Only HWE if you're 100% sure you won't end up owning it
If not, dry clean with full explanation to customer that results are limited etc etc..
Sometimes they'll tell you to HWE it anyway, i never get signed disclaimers but you need to make the customer tell you to do it. offer the hwe, explain its superiority and also dangers but retract it saying you'd prefer not to, get them to talk you in to it so you've put the responsibility back with them.