if the fabrics not pre-shrunk (especially cotton) there's a fair chance it will shrink (by more than the 3.5% that is considered standard)
The most noticeable areas will be the endges of cushion covers, and if there is any lose trim on the sofa itself.
If the covers are loose, you could take them off and hand it over to a dry cleaner who would be able to clean then thoroughly without the added risk. i run both dry cleaning and carpet cleaning businesses and if it tells you not to wet clean then its either a fabric or dye issue. Unless the manufacturer hasnt tested the fabrics properly and has just put the dry clean care label on to protect themselves.
ive seen the prochem dri-pro stuff and its pretty pants to be honest.
If you do decide to wet clean it then i'd get the customer to sign off the risk.