Eating street food is risky. My relatives from Taiwan won't even eat the street food in mainland China for fear of getting sick. Note: I already got really sick from eating the street food in Taiwan.
As a student I'd estimate a good 40 - 50% of what I eat is street food, and it's never given me a problem. You just gotta use some common sense. Like if two street carts right next to each other are frying things in a vat of oil, go for the one which looks hotter. Look for a crowd around a stand and a lot of turnover.
Wheat-based dishes are what Beijing does best. MTGL gave you a good list so here's some more:
La mein - hand-pulled wheat noodles in hot broth, served with scallions, cilantro, and your choice of sliced meat; horse is my favorite. Vinegar and hot pepper paste on the tables to taste. Even at cheap, dirty places the quality of the pulled noodles is really good; they have a nice chew that dried noodles don't. 6 - 10 RMB
Liang mein - now that it's summer the vendors of cool noodles have started to come out. Just simple noodles tossed in sesame oil, sesame paste, garlic sauce, and hot sauce with shredded cucumber and carrot. A good light summer lunch. 4 RMB
Jiu cai he - "Chive box", a dough wrapper stuffed with a garlicky chive filling, wrapped up in a burrito shape and griddled on both sides until crispy. 1 -2 RMB
Shao bing - a round, leavened, baked bread similar to an English muffin in size and shape, but a little bigger. You pick the filling from a variety of steam table fillings which might include shredded potato, fried egg, duck breast, or ham. Comes with hot and hoisin sauce. 2 -3 RMB depending on filling
Uighur/Chinese food - a lot of Uighur ethnic minorities run restaurants in Beijing. You can usually spot these places by looking for Arabic lettering in the restaurant sign. The menus here are lamb-heavy and you can get some delicious hot flatbread served alongside more traditional Chinese food. 20 -30 RMB per person if he goes with a group.
Turkish Sandwiches - I don't know the real Chinese name for these but these are giant vertical spits stacked with dark meat chicken cooking in a gyro-style brazier. They'll slice a portion off the crispy outside and mix it with shredded lettuce, zucchini, and sweet sauce before stuffing it in a shao bing. 4 - 5 RMB
Above all tell him never EVER eat a hot dog in China!! I've been tricked so many times...