Intro And Price Question

What time period is he most interested in?

Roman coins from Constantine I's reform through Valentinian III (about 315-430) are generally very common. Nobody makes good fakes of the common late bronzes, or if they do, they are of a woefully bad quality and clearly brand new. You can build a large collection quickly and cheaply, but for a newbie who doesn't know what is rare or common, I would recommend buying nicer individual coins for less than $10 each. Here is a nice set that you can get of Constantine:

1. Any of the VOT series (XX, XXX) http://www.tesorillo.com/aes/072/072i.htm
These were issued to commemorate a certain number of years in office, and are very common and cheap in good condition.

2. Constantinopolis http://www.tesorillo.com/aes/015/015i.htm
A commemorative issued by Constantine and his sons to celebrate the new capital.

3. Urbs Roma http://www.tesorillo.com/aes/072/072i.htm
This was the counterpart to celebrate the "old" capital

4. Divo Constantino http://www.tesorillo.com/aes/017/017i.htm
This was a posthumous commemorative issued by Constantine's sons, showing him riding a quadriga chariot and accepting salvation by reaching for the Hand of God above.

If you want some older coins:

1. Claudius issued a small coin called a quadrans featuring a modius of wheat, illustrating his promise to undo the damage that Caligula did to Rome's food supply by his wasteful spending. These can be had for less than $10 in decent condition.

2. You can get a worn but identifiable denarius of Trajan, Vespasian, Hadrian, Antoninius Pius, and Septimius Severus for less than $20 each. Marcus Aurelius and Commodus are out there, but run more expensive since they are more popular following the release of Gladiator.

3. The span of about 240-284 is known as the "crisis" era, marked by rapid inflation and debasement of the silver coins. The Antoninianii (double denarii) of Gallienus, Claudius II, Aurelian, and Diocletian are extremely common and cheap.

After only 2 1/2 years in the hobby (ancient Roman denarii), best advice: Buy from established dealers who guarantee what they sell and avoid ebay. Start here:
https://www.vcoins.com/en/coins/ancient-2.aspx

Value is determined by rarity, demand and eye-appeal. The last takes some getting used to, but as you collect you'll see that artistry varied between die engravers. A common but beautiful coin will be much more expensive than the same coin done by a less-experienced die engraver.

As for slabbing, as orfew writes, there is no guarantee. Besides you can't touch a slabbed coin which kind of defeats the purpose of collecting the things if you ask me.