If your fiancée intends to take IELTS for her visa application she needs to secure a score of 4 for the Speaking and Listening modules.

If she is granted a Fiancée visa she will not be allowed to work until she has been granted FLR(M)

Which is basically a spouse visa. Only then will she be allowed to work.

In general there's no mandatory requirement to pass at a higher level in order to secure employment - unless she wishes to be employed in certain specific sectors (nursing for example).

Besides which, she could study and take the needed higher level English test here in UK if required.