Sometimes when a patient has a bypass and on a pump for oxygen there can be some memory problems for a short while after the operation, and maybe the experience your father-in-law had with oxygen deprivation is similar as both conditions are related to lack of oxygen to the brain. Usually temporary but can be some permanent damage of brain cells. .
I did a little copy and paste for you in hopes that this may enlighten you. There are many sites on the internet regarding this condition.
Traditionally, aphasia suggests the total impairment of language ability, and dysphasia a degree of impairment less than total. However, the term dysphasia is easily confused with dysphagia, a swallowing disorder, and thus aphasia has come to mean both partial and total language impairment in common use.
Depending on the area and extent of brain damage, someone suffering from aphasia may be able to speak but not write, or vice versa, or display any of a wide variety of other deficiencies in language comprehension and production, such as being able to sing but not speak. Aphasia may co-occur with speech disorders such as dysarthria or apraxia of speech, which also result from brain damage.
Aphasia can be assessed in a variety of ways, from quick clinical screening at the bedside to several-hour-long batteries of tasks that examine the key components of language and communication. The prognosis of those with aphasia varies widely, and is dependent upon age of the patient, site and size of lesion, and type of aphasia.
I would suggest speaking with his doctor more on this. Take care, Ally