One (of you) is the thief.
One is the thief.
One person within the group is the thief.
Only one of you is the thief.
'One' functions as the subject, and 'of you', as its modification. Modification doesn't agree in number with the verb. Your colleague may be thinking of correlative conjunctions, (n)either . . . (n)or; e.g. Neither you nor I am the thief. With those constructions the verb agrees in number with the closest subject.