Time matters more than distance, but 1 km is a very short ride. She shouldn't have a lot of pain the next day. Either she was wearing poor clothing, the saddle shape, or bike fit are really bad for her, or she hit a bump hard, or some combination of those things. Or she doesn't really want to ride.
First find out if she WANTS to ride. If she does, make sure the bike fits her. Unless she has been very inactive up to now, the fact that her arms hurt after only 1k would lead me to suspect the bars are too low or too wide, saddle nose is tilted down too much, the stem too long, or the frame is too big. Saddle fore/aft position should not be used to adjust the reach to the bars. Fore/aft adjustments are to position the rider's lower body properly in relation to the crank. Get her some padded shorts and chamois cream and instruct her how to wear them; i.e. no underwear. Possibly try a cushier saddle. Problems with cushier saddles generally don't manifest until one starts doing longer rides. Make adjustments she requests. These may be pretty frequent at first. Until she's riding at least 10 miles a few times a week don't worry about trying to find the perfect saddle for her.
Also note, bigger, wider, softer saddles are NOT better for all women. Nor is woman's saddle always best. Myself and a friend are two who are most comfortable on narrow, hard, "men's" saddles. Many women I know who ride regularly prefer men's saddles. Some women are most comfortable with a cut-out. Others find cut-outs extremely painful. I've also known women who weren't comfortable until the nose of their saddle was tilted down significantly. Shorts/saddle combination can also make a huge difference. There isn't one style of saddle that's best for all of us. It comes down to differences in individual anatomy, as well as shorts and riding styles. The Team Estrogen forums are a great resource to help women find saddles that might be comfortable for them, but ultimately saddles need to be tested for awhile. If you don't have a shop nearby with a good saddle try-out policy, eBay can be a way to try out different saddles for an extended time.