You’re allowed to bring in a quantity that is reasonable for personal use, which is usually interpreted as ~1 case. I’ve brought a case of beer back from Germany in the past without a problem. Just don’t plan on having it in your carry on luggage.
I’d look for beer in cans, since it’s easier to carry lighter, and less apt to break, but that is limiting.
I’d have her get a couple different Alt biers and a couple Koelsch biers (something other than Reissdorf or Sunner might be nice, since they show up in the US, Gaffel). They are both styles that are somewhat hard to find in the US. I’d try and get an Alt or 2, first, since a more interesting beer. Diebels is the mainstream one, so I’d try to get a different one, but it is still a fine beer. Uerige is often found in the US, but a fresh one tastes much different than an old one you find warm on a shelf in the US. One of the rarer ones would be cool.
You should be able to find some decent Pilsners or Exports around Bonn, but I’m not familiar with that city. A good Pilsner, in a can so it isn’t skunked, would be an excellent choice.