Romans used both Greek and Latin. They represented numbers using the Roman Numeral system, but, from what I understand, that uses Latin characters. So if a Roman was writing in Greek, how would he represent a number?
Why would a Roman writing Greek represent numbers any differently than a Greek writing Greek?
He'd use Greek numerals
edit: To add at least some support to my claim, here's a Christian inscription from Rome.
It reads "to the sweet wife Argyris her husband Trophimos; she was living 36 (λϛ) years".
Even this example of Latin written in Greek letters uses Greek numerals:
ικ κο[ν]βιξιτ βενε βι[βος] κουν κοιουγ[ει] ετ φειλιβους σο̣[υεις] αννως λζʹ δηφουνκτους αννωρουμ νʹ[․ʹ] μησουμ ∙ γʹ διου[μ — —]
hic convixit bene vivos cum coniugi et filibus suis annos λζʹ defunctus annorum νʹ mesum [=mensium] γʹ dium [=dierum] ...
he lived 37 well-lived years together with his wife and his sons and died when he was 50 years, 3 months, ... days of age.