Your father-in-law is bound to be anxious knowing that his blood sugars are that high, and having high blood sugars will sap his energy too. I don't know what advice he has been given about diet but the foods that increase blood sugar the most is bread, food made with flour like scones, cakes, pastry, potatoes, fruit juice, lots of fruit and dried fruit, bananas, processed cereals, and drinks high in sugar. To start with eating vegetables, especially non-starchy vegetables with meat, poultry, fish, eggs and nuts, will bring down blood sugar and then your father-in-law and he can use the blood sugar monitor to how certain foods affect his blood sugar because the same food can affect people with diabetes to various degrees so that one person finds potatoes (especially mashed potatoes) affect them more than bread and another person will find the opposite. Wholemeal bread is better than white and raw fruit is better than cooked fruit. An apple eaten with meat or cheese, because meat and cheese contain fat, does not raise the blood sugar so high as eaten on it's own. Because your father-in-law has probably had diabetes type 2 for a long time his blood sugars will have been high for sometime, and tablets will take a couple of weeks before they start working properly, so expect to see his blood sugars come down slowly. If he is stressed this will increase his blood sugars too so let him know that he should be relaxed about it. If he gets ill and has a fever he should get in contact with his health care provider and drink plenty of fluids.
Eventually he will find his blood sugars go to below 7.00 mmol/l before breakfast and be less than 9.00 mmol/l 2 hours after eating, and he will feels so much better then. Get him to write down a list to ask his doctor/nurse when he next sees them but meanwhile there will be lots of people on this board that will answer your questions. Remember at his age it is a lot to take in so give him some time to get his head around it. I am still learning five years after diagnosis.