We successfully did a similar application.
We provided as much proof as possible with the visa application.
Focusing on how she would fund the trip, the fact we would support her financially (if needed) and also provided proof of reasons for why she would not overstay.
I.e. « Sally » is a full time student at Bippty Bop University in La la land where she is in her third year of studying for a degree in xxx. Term starts again on xxx September and she will return to xxx by xx date in order to continue her studies.
We then mentioned the length of their relationship and the fact we already had a good relationship with her and we were very supportive of their plans to marry in the future.
We also mentioned plans for the trip such as « Sally is coming to England in order to attend the wedding or our daughter Jane. We consider her part of the family are excited to have her be in England for this important event etc ».
We encouraged « Sally » to submit as many supporting documents as possible in addition to our letter of invitation and support.
For example, showing we had a good income so could support her if needed during her stay, proof of good travel insurance for health needs, proof of her ongoing commitment to studies in another country.
Basically trying to show that she will not overstay, she has sufficient financial means and has good travel insurance.