TiVo requires monthly, yearly or lifetime (of the machine, not the owner) payment for service. This service includes guide data, which is brought in by TiVo from King Media Service, and periodic software upgrades for the TiVo itself. (It also requires connection to either a phone line or the internet.) Without the service, the TiVo is essentially useless. The software updates are just that, but the guide data is the lifeblood of the TiVo experience. In addition to providing a channel guide to the user, it also enables all the search functions and, if you choose to set advance recordings, the guide data is how TiVo knows which channel to record and when.
There are several different TiVo models.* If his cable service is directly off the wire, as you describe it, with no converter box of any kind involved, you need to be sure to get a TiVo with Over The Air (OTA)
capabilities. Even though your father's service is likely coming from a cable provider, the fact that his connection does no require a decoder box means that TiVo will see the signal as an antenna --or over the air-- source. Some TiVo models ONLY work with cable cards, others ONLY work with DirecTV, etc. If the model doesn't specifically say OTA, chances are it will not work for you.
*Some very old TiVo models were able to partially function without a service plan, and you can sometimes find these old machines on eBay or the like. By partially, I mean that without the guide data, you could still use them just like the old VCRs: Make recording on this day, start the recording at this time, end recording at that time, etc. Might as well just get another VCR instead.
Edited to add: The above is very much an overview. I've been using TiVos since 1998, and can provide you with a lot more information if you want it, especially if you have specific questions.