If he is just starting arena:
1. Not enough speed;
2. Tried to beat someone in challenger/master;
3. Went against a hacker (yes this is a thing).
Also whenever you reset the arena the game will usually give you 4 easy opponents and 1 which is impossible (ex. people with ~4000 points in Silver) so as to not give you that consecutive bonus.
Regarding hackers in arena, first I also thought it wasn't possible but then a friend came to me asking for help with his arena team to get out of Silver (he was Silver V) so i did the same thing i did when i got to gold the first time built his units with about 150-160 speed which is a good amount for getting into Gold, yet at some points we went against teams of level 15-20 people who outsped him horribly (as in they were getting 3-4 turns in before he even got to act even tho they had no CR push) while at the same time dealing 6k-10k damage with everything they used, which means they cheated to get gear they should not have at that point.