I found this answer to a similar question on the New Advent website, here. Just scroll down a bit to find it.
"My daughter recently married a Seventh-Day Adventist (against my husband’s and my intense opposition). He attacks the Catholic Church whenever he gets the chance. One of his main objects of scorn is our Holy Father, who, he says, is the beast spoken of in Revelation 13. He claims he can “prove” this because the pope’s title, Vicarius Filli Dei Latin for Vicar of the Son of God), adds up to 666–the “number of the beast” mentioned in Revelation 13. Is this true? How can we respond?"
"You were right to object to this marriage. No doubt, with his attitude, your son-in-law will do everything he can to rob your daughter of her Catholic faith and wrest her away from the Church (this is why Paul warned us not to be “unequally yoked” [2 Cor. 6:14]. Encourage your daughter to remain steadfast in the faith, and do your best to respond to her husband’s attacks in a charitable but firm way.
Let’s consider this accusation. Latin, Greek, and Hebrew have numerical values assigned to various letters in their alphabets. In Latin the values are: I=1, V=5, X=10, L=50, C=100, D=500, M=1,000. By extension W=10 (because W=VV, or two Vs together), and U=V (because there was no letter U for the Romans; where you see the letter U in modern writing, use the letter V instead).
As you can work out for yourself, Vicarius Filii Dei does add up to 666 in Latin: Vicarius=112; Filii=53, Dei=501. (Ignore letters which are not assigned a numerical value.) The problem is that Vicarius Filii Dei is not a title of the pope. One of his titles, in fact his chief title, is Vicarius Christi (Vicar of Christ), but, unfortunately for Seventh-Day Adventists and other anti-Catholics who attempt to use this ploy, Vicarius Christi adds up to only a measly 214, not the infernal 666.
Since the average person, Catholic or Protestant, hasn’t the foggiest idea what the pope’s titles are in Latin or English, anti-Catholics (some of whom know better) can get away with this subterfuge.
But what if one of the pope’s titles did add up to 666? Would that coincidence prove the pope is the beast? Hardly. It would prove nothing because lots of names and titles add up to 666. By using a nifty little technique you can force a Seventh-Day Adventist to admit that the addition to 666 proves nothing, even when it’s a papal title that’s in question. Here’s how.
Have your son-in-law do the math, and he’ll be shocked to learn that the name of the woman who started Seventh-Day Adventism, Ellen Gould White, adds up to 666: Ellen=100, Gould=555, White=11. Ask him whether this “proves” that the foundress of his religion was the beast? If he says “No,” then the tallying of the name means nothing. If he says “Yes,” then what’s he doing belonging to a church founded by the beast? Either way his argument collapses. (Isn’t apologetics fun?)"
If your friend is SDA, then this answer is even more appropriate for that situation. Either way, it confirms the fact that trying to “prove” that anyone is the “beast” of the Apocalypse by assessing the “number of their name” (or title), is a futile endeavor. There are probably hundreds of names or titles that you could make fit the bill, but it doesn’t prove anything. It’s just a waste of time and energy.