From my reading, technically your dad is correct in that leaving device with a full charge all the time does degrade the battery. This is because the voltage increases as it nears 100% charge.
However devices these days are very good at managing charges, so I personally am not too concerned (but I do avoid leaving it plugged in all the time). What I do worry about is temperature, that will degrade my battery.
Keeping Your Laptop Plugged in All the Time Will Kill Its Battery Faster | Gadget Lab | Wired.com
Microsoft's battery care tips say:
"•Operating temperature: Surface is designed to work between 32°F and 95°F (or 0°C to 35°C). Lithium-ion batteries are sensitive to high temperatures, so keep your Surface out of the sun and don’t leave it in a hot car.
•Recharge anytime: The battery doesn’t need to be empty or low before you recharge. You can recharge the battery whenever you’d like. However, it’s best to let the battery run to below 10 percent at least once per month before you recharge it."
Microsoft Surface Pro and Surface Pro 2 battery and power | Recharge Surface Pro