Recommend that your client get an appraisal (of course, from a certified appraiser). This usually will help the seller--after having worked through his/her/their feelings of denial--to accept the reality of what the house is worth now. You could also take the seller around to several (10-20) properties that are similar and priced right.
If that doesn't help, then perhaps some more time on the market and a few well-priced offers will help the seller to accept reality.
Nevertheless, if your client still refuses to accept reality, then invite him/her/them to attend one of the local REI meetings with you. There, the seller could discuss the property with some of the investors to get an idea of what they'd offer to purchase the property.