Unless your loved one has poorly controlled diabetes, it is not recommended to restrict their diet.
The reason for this is that older adults are already at a high risk of developing malnutrition, and further dietary restrictions are more than likely to have a negative impact. All older adults with diabetes (and without) should be following a well-balanced diet including fruits, vegetables, wholegrains, and adequate amounts of protein. Having one plain biscuit with a cup of tea should not affect their blood sugar levels too much. Making homemade biscuits allows you to control the amount of sugar that is in them as well as allowing you to increase the protein content. Using almond meal or Greek yoghurt are a fantastic way of increasing the protein content of the biscuit and lowering the glycaemic index. Alternative snack options include a savoury-style cracker with ricotta cheese or tomato and cheese, or cut up fruit with yoghurt.
If the biscuit is what is bringing the individual joy, then leave it alone. If their blood sugar levels are poorly controlled, it is important to look at the food intake over the full day and test blood sugar levels two hours after eating to help guide dietary changes. It is always helpful to see an accredited practising dietitian in the community for ongoing guidance.