If this intangible property is considered community property, then I suppose so. But was there any existing basis? There could also be a step down in basis, depending.
They should probably get a professional appraisal. For a widow who is not a tax practitioner, selling a tax practice (client list) of their deceased spouse may not be an arm’s length transaction, so true FMV may not be known.
My point was, as a non-tax-practitioner, the widow probably had no choice but to sell. Her husband, if still alive, could have turned down a deal if he thought he was better off retaining the clients, but she does not have that option. So maybe the FMV should be higher than what she sold it for.