I think dressing athletically casual is fine - a neat/clean appearance (hair up, makeup not overly done, etc) is important too, I think. Just tell her to be herself - whatever makes her feel comfortable.
Always better to be overdressed than under IMHO. Whoever you're meeting with will appreciate it (coach, client, employer etc.) regardless of what they're wearing. One suggestion if she's wearing athletic wear...know if the team she's meeting is sponsored by anyone. Ie, don't go in covered in Under Armor logos if Nike sponsors the team.