Very Random, Very Fuzzy
This is just a fun exercise.
Lune Lace (Thanks) shared some photos that seem to show that the person on the lift beside Pax has a similar chin - jawline as Brad's.
The person also may or may not be wearing knee support on his left knee. Brad told People he injured (tore?) his ACL four years ago.
We already noted that the person and Angelina are sporting identical helmets - Giro Nine in Matte Dark Shadow. No one else in her party had the same identical helmet.
It is also interesting that this person had the strap of his goggles inside his helmet. The goggles can't be accidentally torn off and he doesn't intend to lift the goggles off his face -- a hint that he wants to keep his face covered. Almost everyone else including Angelina had their straps outside their helmets.
Maddox's goggles have a mirrored orange tint and his helmet is black with black straps.
A friend of Maddox would be with Maddox and not Pax. They are riding a quad chairlift. If they were going up the mountain as a group of friends, four could fit in that chair. Pax is clearly the only kid on that chair. The build and height of this person is similar to that of the other adult on the lift. Pax is going on a run specifically with this person. The other man may or may not be with them.
Pax has an athletic build and his frame is not too far from that of the older boy who accompanied them on the plane, but he is appreciably slighter than this person.
There are old photos of Brad teaching Maddox to snowboard and on a ski lift with an instructor.
Neither Brad's friend nor Angelina's source have given one word about Brad and their New Year's holiday. Their uncharacteristic reticence can be taken as tacit confirmation that Brad was with them. So we shall just go ahead and call this person "Brad."