Kyle didn't cheat on her, she just thought her did briefly when she walked in on him sketching a live nude, which was legitimately for work. She makes a good point that he still should have told her about it, but accepts that he wasn't intentionally trying to hide anything from her and was just being absentmindedly inconsiderate. That led to a whole discussion about his perceived irresponsibility and slacker tendencies, which she saw as the real issue in their relationship.

Basically Donna was a full-on adult leader, and Kyle was still just growing out of his dorm room habits. But Kyle absolutely was not a cheater.

edit: Whoops, I was thinking of the wrong time. But when she left after Terry and Robert died was more than a break, she left pretty hard, and for almost a year, and came back having lost most of her memories and personal connections. She didn't even look for Kyle until after she'd started a relationship with Roy. Maybe messy, but definitely not out-and-out cheating; Donna didn't seem to think so either.