The PC joined the campaign at 6th or 7th lvl, so he came in with the alienist PrC already obtained.
Here's what an alienist loses over the 10 lvls (in comparison to a regular Wiz):
2-3 bonus Metamagic or Item Creation feats, familiar progression, 2 pts of Wisdom, develops a phobia towards a type of creature, has to take Alertness as a prerequisite feat.
What he gains:
2 Metamagic feats, +1 insight bonus to all saves, +6 hp, all summoned creatures and familiar become pseudonatural, a free spellcasting slot at highest level (only usable for Summon Monster spells), immunity to aging, becomes an outsider.
Not drastically more powerful than a regular Wiz, but the gains do outweigh the losses. YMMV.