Ensure you human friend is always on fire. Don't look at them.
This prevents any Vampires from looking at them or, generally, being in their presence. The difficulty to avoid Frenzy in the presence of a lit cigarette is 3. Even if an elder vampire somehow got to roll 20 dice on the courage check involved, it would fail >99.9% of the time in the presence of ~600 cigarette-sized fires, which can be fit as distinct units within a ~225 mm diameter circular housing (e.g. a short length of pvc pipe). Care must be taken to avoid allowing the fires to merge into a continuous flame, as that reduces the probability of failure considerably.
Provided a proper fire-based mechanism is constructed, no Vampire can directly interact with your mortal friend without spending willpower each round they are exposed. Almost all Dominate/Persuade powers require direct eye contact so this protects against almost all attempts at control via those methods, regardless of the opponent's power.
That said, there are exceptions that need to be addressed:
Summon works on anyone the user has ever met. This means that, if anyone has met your friend, they can summon them to themselves across any distance regardless of fire. In order to prevent against this, you'll need to ensure that an appropriate task force travels with your companion to their destination.
Note that Summon, even at the highest levels, cannot make anyone do things that they think are dangerous--Terminal Decree applies only to Dominate powers. That is, your companion will book a plane flight to their summoner's destination rather than trying to swim across the Atlantic, and will book hotel reservations and security accommodations on the way as per normal for them unless doing so slows them down. Since they know where they are going, having agents already at that location travel to the exact location of the summoner, light said summoner and the surroundings on fire, and subsequently mounting their head on a pike should discourage any future nighttime visits. Note that your friend knows perfectly the exact location of their summoner at all times until the next dawn and a summon, once given, cannot be revoked. Most vampire v. hunter tactics, even at the highest levels, require the vampire to be ambushing the humans rather than the other way around to have a decent shot at success-- knowing exactly where the vampire is at all times makes that pretty difficult, particularly since Vampire combat abilities rarely have the sort of range that human weapons boast. And the DC for 'trapped in a burning building' is 9.
Works across live video feeds with Star Magnetism. Allows 'small requests' to be made of the target. You can't really stop this, but you will have video of them requesting it and the requests have to be things the target considers not a big deal.
Lets all domination powers no longer require sight. Instead, touch is used. Care must be taken that powerful shapeshifted vampires not be able to touch your friend while avoiding the fires. Ultimately, if there are enough primogen-level vampire lords willing to risk probable death in an attempt to control your friend, they will eventually succeed with this discipline. Your job is just to do good enough at perimeter sweeps, etc, that the first dozen or so die. If there are more ancient vampires after your friend than people protecting them, you are kinda screwed anyways.
Just make sure they don't make oaths. Problem solved.
If your friend isn't the descendant of a vampire, you're good. If they are, those specific ancient super vampires they are a mortal descendant of can use Dynastic Power on them. This only works for a maximum of 6 generations without inbreeding, so you have a fairly limited list of people to check out.
This requires the vampire using it know your friend's location. Unless your friend recently foolishly used Summon (they didn't. They are a mortal), it should be pretty simple to conceal their location from vampires. Vampires notably lack any powers that let one quickly track down targets over country-scale ranges. The level 6 Auspex power Horoscopic Forecast lets an extremely powerful vampire, with a lot of work, divine a target's future location with no assurances as to when in the future target will be there, save that it will be within the next year. That's an example of what vampires would have to resort to to get your ally's location via stay-at-home magic power use, which is why in practice they will do it by having minions try and figure it out by searching the internet and physically searching probable locations and places that might have clues.
If a powerful vampire does get your friend's location, however, far mastery will doom them.
Carry the Master's Voice:
Don't let your friend talk to vampiric thralls. You can ID thralls with Auspex. Problem solved.
And that's about it. Of course, there is also Plot Device, the generic level 10 power for every discipline in Ghenna. You can't stop that from working, unless you also use Plot Device. Or possibly even if you do. Or maybe you can stop it, easily. Or with effort. Basically, the power just does whatever the ST wants with whatever limitations the ST thinks would be fun, so it's not really relevant to actual plans.