My exMIL was doing this to my exFIL for a good while. He has an incurable disease that prevents him from being as mobile or able to fend for himself as he might like to be, plus I suspect he couldn't boil an egg even if he tried, and she was determined to remain at the weight she was when she married (she was very proud of having avoided middle age spread). ExMIL is also a vain and domineering Narcissist. If exFIL tried to help himself to a snack (not brought up in the age of snacks so this didn't come naturally to him) he was bossed out of the kitchen. After years and years of being bossed and henpecked at home, this is what a very intelligent former surgeon was reduced to. He accepted this because this is how exMIL was for their entire marriage -- In Charge Of The Kitchen and not afraid to shoot from the lip.
ExMIL didn't like yogurt so didn't buy it. She didn't like eggs so never cooked them. She bought crackers, pretzels and Coke for the grandchildren.
The result was serious weight loss for exFIL that no-one dared to explain even though all the exILs used to complain that they would go to MDs after the weekly obligatory Sunday dinner at exMIL's as they never got enough to eat chez exMIL...
So exMIL and exFIL went to the Mayo Clinic at the urging of oldest exBIL, who is a doctor himself (they live in the US), where after lengthy questioning and tests, exMIL was told to feed exFIL more. Turned out that no breakfast but a cup of coffee (because exMIL is not a breakfast person) followed by a sandwich for lunch, then dinner consisting of half a potato each, plus one small fillet of fish each and a few leaves of salad was not enough. 
OP you must intervene. Whether they have a housekeeper or whether you are afraid of your mother or not, your father could end up being starved to death or dying of some simple illness that he simply wouldn't be strong enough to fight.