Does he just want to play back, or play and record?
You have several choices depending on what he wants to record, and how difficult the operation can get. It also depends on the type of TV he has.
Let's start with the choices from simpte to complex.
1: playback only
2: record only the remaining analog channels available on the cable. This would normally be the local network channels. This connection would still let his timer recordings work.
3: record all digital channels. This connection will break his timer recordings, and he would only be able to record and watch the same channel at the same time, unless you add a 2nd box, or a small DTA digital adapter.
Let me know on what choice would work best, and then we will go from there.
If he has a recent TV with multiple inputs, then this is simple.
Take a set of yellow/red/white composite video cables from the output of the VCR to one of the composite video inputs on the TV. Just match the colors, and then select the proper input selector on the TV's remote to choose the input for the VCR. He will then be able to play back his existing tapes. As an example, if you hook the composite cable to video 4 on the TV, then choose video 4 on the input selector on the TV's remote. If you don't have a composite video cable, they can be found at Wal-Mart in the electronics section for a few $$. Do not use a coaxial cable from the output of the VCR, the VCR coax output will stay disconnected.
This does not involve the Comcast box, and does not interfere with his existing Comcast wiring betweek the set top box and the TV for watching TV. He just needs to change the input selector on the TV between watching live Comcast TV and the VCR.
If he want's to record, then this playback connection will stay the same, and we can work on a recording solution.
I bought my first Panasonic VCR in 1982 @ $640.00, which had a corded remote, was top load and had a 12 preset 35 channel cable ready tuner and video outputs. Around that time tapes were over $10 a tape. The heavy old thing is still in the basement sitting on a shelf. Here's a shot of the back panel where this old clunker has a video and audio output.
I'm not saying there wasn't any without video outs, I just don't remember seeing any. There were probably some real cheap ones that save a few cents be removing the jacks. The RF modulator on the VCR would need a video level signal to feed the modulator, so the circuitry was there anyway. Of course, those old ones were mono, so only one video and one mono output.
Hopefully your Dad's VCR has a video output, if not then please post back for alternate connections or any more questions.