If one has CHF and walking makes them tired, better to scale back generally, or make effort to walk constantly? For my 91 year old grandfather.

CHF. Exercise is generally good for folks with chf, but let's be realistic. A 91 year old who has (even) well treated CHF is not going to be able to do much! 5 minutes of low level exercise (walking or swimming) once or twice a day would be as much as one could expect. He should be encouraged to do whatever he feels up to doing and there's no point in over-doing it.
Rest. Better to rest, but more importantly, have follow up to be sure meds are appropriately helping maximally - especially if legs are swollen, or difficult breathing, or swollen neck veins when lying back slighltly.
Exercise. Exercise has been shown to be of benefit even with heart failure patients such that guidelines suggest having such patients enroll in cardiac rehabilitation programs where the patient is monitored while exercising.