Teams based around one or two types usually fail since they have no synergy and share common weaknesses.
Your friend should understand that each typing has its own unique advantages and disadvantages based on the metagame in question. As for as which types are better than others, a skilled battler knows that it takes a combination of different typing to build a successful team. So no, it's not unfair at all to build a team around a certain type. It just makes the team building process a bit more difficult.
You can bring this discussion into the Strategies and Movesets DCC if you like.
*closed*