I did both for 'O' level and enjoyed them both.
I don't think you can say one is easier than the other - but if she has already studied some French but no German, then it may seem easier to take French; at the very least she willbe familiar with the language. However, she may want or prefer a change - of teacher as well as language. There is no right or wrong answer.
As to what would be best - well, Chinese would be very useful given the way the Chinese economy is growing and the increasing influence the country will have on global affairs.
Of French and German, however, it may be that French is more useful given the number of French speakers/French speaking countries around the world compared with German speakers and German speaking countries.