If it's mainly for use at home, 17" is OK, but it's somewhat big and heavy to take with you taking the underground to and from your work everyday.
Personaly, I have a 15" Dell Precision laptop from work, but I and most of my colleagues use it at work with a separate monitor (4:3, not 16:9), and a (wireless) mouse and keyboard. so that laptop is used as just a small desktop that you can take with you to a meeting or home (and, yes, I use it with monitor, mouse and keyboard also at home).
Using a VPN to work is just software and works on all PC's or laptops, provided work offers it.That's a question for the IT dept.
Using Excel (and mail and browser) is only a basic task for any modern laptop, so quite unsuitable as exclusion criterion for any model. But he might prefer a 128 or 256 GB SSD above a 1 GB HD, and that makes it somewhat more expensive. Presence of at least 2 USB3 ports and HDMI out can be considered standard nowadays, so that will be easy.
If it's for work, it's pleasant to buy a laptop that comes with a 3 year next working day tech service at home or office. The usual consumer grade laptop - when something is wrong and it needs repair - easily takes a takes a few weeks to get back.
My dad would like to do some of his work at home. He uses mostly excel, but might need to use some sort of virtual link to his office computer. I think a larger screen would be better- at least 15in but probably 17 would be best. Looking for something between 400 and 800 bucks. Any suggestions?