Generally, slower quad > faster dual due to multi-tasking. You would want the faster dual (and depends on how much faster), if your dad was working with a specific application that he wanted to get something done faster, and it wasn't fully optimized for multi-threading. Some applications were optimized for just a single core, but nowadays, it's 2 cores, or 4 cores, or sometimes all cores.
For example, Excel utilizes multi-threading, but some operations aren't, like sorting (uses a single core). Anyways, I would just recommend the quad core over the dual core anyway. You could always set Excel to run at most, 2 cores for all operations, then have 2 cores used for all other applications for multi-tasking. Multi-tasking is the key word. If you had a single core, and it was tasked with something, you wouldn't be able to do anything else until that task was finished. With more cores, you can work on other things while the first task was still running (as long as it wasn't utilizing all the cores).