If someone elderly thinks they forgot their lisinopril, should they take a second one or just follow BP readings?

Move on to the next. Forgetting a single dose of an antihypertensive agent, particularly a drug like lisinopril, is unlikely to lead to adverse consequences. However, there is inherently more danger in "overdosing" on a blood pressure medicine since they may lead to fainting/lightheadedness resulting from excessively low blood pressures. This may lead to falls and actual physical injury.