The grass simply can't compete with the oak tree for water & nutrients. St. Augustine's grass does well under oak trees (in Florida). Consider pots or planters around the base of the oak tree, a garden bench, decorative mulch. Many trees won't tolerate having their roots disturbed.

There does appear to be rotting at the base of the tree though, did your friend alter the soil level when they laid the turf?

It does look as if the root flare which forms above ground has been filled in with soil - certainly the grass should not be so close to the tree anyway, and if they did add soil, they should remove it and the grass from that area asap...

Thanks all, I’ll ask more detail what they did and cut a perimeter round the tree. With abit of care will that part of the tree improve over time? Thanks!

Maybe - they need to check that area after its had a chance to dry out when everything they might have added has been removed.

Looks like the effects of a lightning strike on the tree which occurred sometime in the past. The soil around the perimeter of most trees is hard and compact and turf set close to it cannot get the roots down far enough for a good grab into the soil it lies upon. Also take into account the shade around a tree in leaf. Expect a sparse area of grass growth around most trees.