Yes, you are eligible for Eurail. Russia is not (and was never) included in the Eurail/Interrail programme.

Getting over the Russian border is going to be something else though for the moment :/

@BrendanDB I’m thinking of starting the trip from another place. Take a separate train to another country and starting from there with the eurail pass… I noticed that there are no trains to/from Russia currently, so that’s the plan. 
thanks for the reply :))

Good . Better use your Brazilian passport for the pass. If you have any further questions about the European train system, connections or reservations, this community can always help!

Best will be to take a bus from St. Petersburg to Tallinn or Helsinki and start your interrail trip from there. If your student visa allows you to enter Belarus too, then would be also possible to go via Grodno/Brest to Poland.

He can use his Russian Residence proof either for Interrail But yes Eurail with his Brazilian Passport is much easier