look, dont feel bad, it's out of ur control. i dont advise that u try to upstage it though. just be cool and continue to be there for her. take care of her. that's what matters more than any bunch of flowers.
What did the flowers mean, are they friendly or something else? That's a bit unusual. Think of it this way, why would he send her flowers if he didn't think he'd have something to gain from doing so, she could have been giving him a bit of false hope or hasn't mentioned that she has a boyfriend. The flower sender is trying his luck.
Let her be happy with the flowers, if she likes them that much maybe she should just be with the other guy!
As it should make you feel like that, it's not right. Not only does your girlfriend need to understand boundaries and she should've "shut the guy down" herself (to quote @Iron_Man because I agree with him) but also you could talk to him as well and make it clear to him. Also try talking to your girlfriend about boundaries and what should/shouldn't you guys be doing and what's allowed/not allowed because she either would have no problem cheating or just doesn't get how relationships fully work.