Robbed in your sentence is a verb. In the clause who was robbed, "was robbed" is the main verb, and it's in the passive voice.
Sometimes it can be difficult to see whether a past participle is used as an adjective (adjectival) or as a verb (verbal). When participles are used as adjectives, they are called Participial Adjectives,
Adjectives with -ed or -ing endings are known as PARTICIPIAL ADJECTIVES, because they have the same endings as verb participles (he was training for the Olympics, he had trained for the Olympics). In some cases there is a verb which corresponds to these adjectives (to annoy, to computerize, to excite, etc), while in others there is no corresponding verb (*to renown, *to self-centre, *to talent). Like other adjectives, participial adjectives can usually be modified by very, extremely, or less (very determined, extremely self-centred, less frightening, etc). They can also take more and most to form comparatives and superlatives (annoying, more annoying, most annoying).
Most participial adjectives can be used both attributively and predicatively:
That's an irritating noise. This is an exciting film. (attributive)
That noise is irritating. This film is exciting. (predicative)
When participial adjectives are used predicatively, it can be difficult to distinguish between adjective and verbal uses. Here are my own guidelines:
Participle adjectives are about a quality or attribute of the noun it modifies. This explains that why many of them refer to mental states and feelings. For example, She is bored, The noise is annoying, I was surprised, The window is broken. When they are used as verbal, they will be about the action. For example, The noise is annoying me, The window was broken by him.
If the agent, e.g. by-agent phrase, is clear, then the past participle is used as verbal. Compare:
I was delighted to meet you again. I was disappointed to hear your decision. (adjectival)
I was delighted by his compliments. I was disappointed by your decision. (verbal)
Another simple test is to test its 'very' acceptability, i.e. to see if we can add 'very' or not. If it makes sense, it's an adjective, if it doesn't, it's a verb. For example, She was very upset and She was very bored make sense, but He was very robbed doesn't. Also note that though this test works in most cases, there are some exceptions (see Swan's Practical English Usage, entry 410.4),
I was very amused / much amused / very much amused by Miranda's performance.
Also some participial adjectives are not commonly used with very:
That's Alice, unless I'm (very) much mistaken. (NOT ...
unless I'm very mistaken.)
He's well known in the art world. (NOT ...
very known ...)