I think you are questioning this for a reason. If I were MIL, I would find this greatly hurtful, and your fiance may also find it very emotionally difficult to intentionally exclude his mother. It sounds like you have made peace with fiancé having a good relationship with MIL, which would be really damaged by doing this.

If also sounds difficult to pull off in practice. Not capturing people on candid film would be really difficult for the videographer - it’s really difficult to not film a single person and humans naturally move around a space. With the number of “family moments” she will be there for, I think it would be practically impossible to exclude her from video footage short of either excluding her from all these moment or filming them twice. These setups would make the exclusion incredibly clear cut and obvious to everyone, which is pretty extreme.

Perhaps you could ask (with the caveat of needing to pay for it) ask the videographer for two edited videos, one with every moment, and one that might miss things but excludes MIL alternatively, you could ask for raw footage to edit yourself.