Was the ex listed as the grantor? If so, it's an invalid deed because he doesn't have an ownership interest in the property. I can give you a deed signed by me for the Brooklyn Bridge, but it doesn't mean anything because I don't have an ownership interest in the bridge.

If he listed her as the grantor and forged her name, then it's a cloud on the title.

Either way, it potentially is a cloud on the title, and she probably has a legal claim against him for potentially slandering the title to the property because regardless of whether the deed purports to validly transfer an ownership interest in the property or not, it will make it more difficult to sell the property because this weird deed is now in the property records/chain of title.