As I understand the legal requirement is to make reasonable effort to provide facilities. The provision of an interpreter at no extra charge for interpreting the main group's songs is, in my opinion, beyond that limit already.
It's not like a cinema where a recorded tape is simply passed to all places at once and played. This is an interactive task to do with a show that can change direction any moment.
Regardless whether the mother is after a big payout for herself or hoping to change the rules to make unreasonable demands on concert producers it is still wrong. And may have unfortunate consequences.
This attitude of litigation and every narrow opportunity is a sad direction for society to go. And it minimalizes those with reasonable, genuine, claims.