Replacing one compartment of the knee joint is called a unicompartmental knee arthroplasty (UKA). Most often it's the inside (medial) half of the joint that wears down first and becomes arthritic.
There are several pros and cons to this operation. Operative and recovery time are less. The cost is less, too. But there are some concerns, too. Studies show the UKA doesn't last as long as the TKR. Other studies show function is improved more with the UKA compared with the TKR.
There may be an increased need for revision if the one-sided implant comes loose or the other compartment wears out. Then the patient would need a total knee replacement (TKR) after all. Overall, the UKA has become more popular as surgical implants and techniques improve.
A recent analysis of the cost versus benefit of UKA and TKR for low-demand patients confirmed the usefulness of the UKA. Low-demand means the person is fairly inactive and unlikely to put much stress on the new implant. Your father may fall into this category.
If the UKA gets him back on his feet sooner he may become more active. Many older adults find that pain relief from the implant makes their daily activities so much easier. They weren't looking for a game of tennis or to take up jogging again, anyway. Many elderly patients die of unrelated causes before the UKA ever wears out or needs revision.