Well looks like there's rot or frost damage even. Let's start off with some general info. It will make it much easier for me or anyone else to help give advice.

1. What Hardiness zone is the cactus currently in? Just click on the green and it will take you to site so you can easily find this info.

2. Is the cactus inside or outside?

3. What type of mix is it in?

4. How much does this cactus get watered?

Could you ask your dad to take a pic farther back so I can see the whole cactus;).

It’s in hardiness zone 6b (Southeastern CT, particularly) and indoors, far from windows. He said he has “no clue” what soil it’s in and waters it once a month. :)

Well from what I can see it's time to put this cactus on the operating table. The rot will just spread. You can obviously see it already has:(.

To save what's left he will have to cut with a sharp clean knife.

This is where I would make my first cuts.

The area I circled I cannot tell if it's good or rotted. If he cuts up far enough there may still be healthy enough cactus left.

This little guy he can twist off and set on top of some cactus mix. Keep dry so no water, keep warm, and in bright filtered light;). Eventually roots will pop out. It's then he can begin to water and start acclimating to stronger sun.

After he has cut everything and salvage what he can. The cuttings need to form a callus over the fresh wounds. This can take a couple weeks or more. There needs to be a good scab formed over the cuts. This will help prevent rot and any bacteria making a home out of the new cuttings. Just like when we get cuts. During this time the cuttings need to be placed somewhere warm, dry, and in indirect light. Make sure the humidity is low so no fungus can form. If he is worried about this he can sprinkle a tiny dusting of cinnamon on the fresh cuts. Cinnamon acts as a natural fungicide. But most people's homes are dry during this time.

Once a good callus has formed he can set on top of a mix that is 60% rinsed and strained perlite or pumice and 40% cheap cactus mix. Most store bought cactus mixes are loaded with peat. Peat is no bueno for succulents. This stuff once dried out turns to concrete and becomes hydrophobic. If watered regularly it will stay soggy. Peat also compacts around the roots which doesn't allow for proper oxygen to get around the roots. So the roots get choked out.

Now it's just a matter of time for roots to form. Which can take a month or two. Maybe longer since it's winter. Just have him keep the cutting dry, zero water till roots form, warm, and in bright light. Once roots form he can water. Rootless cuttings don't always look that great. So when the cactus perk up he can be pretty sure roots have formed.

Next winter the cactus should be kept pretty dry. All my cactus are kept outside and dry during the winter, but since his are overwintered in a warm home a little water is ok. But I mean a little. I'm talking no more than watering once a month. Even every other month would be fine. I'm sure I'm forgetting something. If you guys have any questions just ask. Goodluck;).

Thank you so so much, he and I will follow these instructions very closely. All the thanks in the world!!