The flu vax is nowhere near 100% effective, it just gives you a chance of not catching certain strains (there's also a very prevalent strain circulating right now which was not included in the vax). There's no way I would chance it since you know that's what they have.
How long have they been "over it"? Maybe they're no longer contagious? I would definitely err on the side of caution with assuming they're no longer contagious though. Maybe even put in a call to your care provider to see what they say.
I'm not at all alarmist about germs, but I would definitely not intentionally expose myself to the flu virus.